Parenting differences... don't want to lose a friend

This is a long one, so get the popcorn!

I have a good friend who's daughter is the same age as my son (both 3yo, Miss is a month younger than my boy).
We used to have multiple play dates a week (all at her house as she doesn't drive) but now I find myself making excuses alllll the time about why I can't come over because I'm sick of the way my son is treated at times.
First of all, my girlfriend does love my son and I know she doesn't deliberately treat our kids differently, but she doesn't keep an eye on the kids. I refuse to leave the room they're playing in, not only because they're toddlers but because her daughter can be quite aggressive and I don't want my son being violet with Miss even if it's retaliation. I feel they need supervision at all times, but my girlfriend says "they'll sort themselves out". I can honestly say my son does not hit, but he will push if he tells Miss to stop pulling him around or sitting on him, and Miss will retaliate with pushing, hitting and scratching. My son always seems to come out of it worse off, and my girlfriend will be straight into my son if he pushes, but says nothing to her daughter and dragging him around by his shirt or sits on him. I still tell him he shouldn't push, but how many times should he have to ask to be left alone before my girlfriend tells her daughter she should leave him alone?

We have spoken about disciplining other people's children. I've been quite vocal about how I'm trying to teach my son he has to respect his elders, and if he needs to be spoken to when I'm not around, I'm ok with it, so if he was ever at her house without me and he does the wrong thing to give him a time out. Now, my girlfriend disagrees. She says it's her job to discipline her child and she won't have anyone else doing it. Fair enough, your kid your rules.
However, she seems to have taken this as an invitation to speak to my child even when I'm trying to discipline him, and I feel this is why her daughter also feels like she can. Her daughter smacks my son when she doesn't like what he's doing, and girlfriend says "she does that coz she's seen you do it." as far as im concerned that doesn't make it ok.
An example being when I was at her house a few weeks ago, my son started jumping on her lounge. I walked over and picked him up and sat him on the floor and told him if he can't sit on the lounge to stay away from it, and my girlfriend crouched down to him and started telling him how naughty it is to jump on someone else's lounge. I feel that I had handled the situation so there was no need for her to put her two cents worth in. Also, while my girlfriend was speaking to my son, her daughter started screaming at me saying "YOU LEAVE HIM ALONE!". I know that my girlfriend would hate it if I told her child that was naughty, so why did she feel that speaking to my son AFTER I'd already spoken to him was more important than speaking to her daughter!?
I also HATE that my girlfriend allows her daughter to yell and curse at me without consequence. My husband and I will not put up with our son yelling at us, so I don't like him to see another child speak to me that way.

Another thing is the sharing. All the toys at their house belong to Miss (of course because it's her home) and I understand toddlers aren't the best sharers, and I don't have rose coloured glasses, I know my son doesn't always want to share, but I believe it's my duty as a mother to teach my son to share. However, Miss doesn't like to share and my girlfriend thinks that's is ok because all toddlers don't like to share and because Miss is a girl and "you'll realise when you have a girl that's some fights aren't worth having."
Example - last week when we were heading over for a play date my son wanted to take his toy phone over. I said before we left, and again before we got out of the car “if you take that inside you have to share it with Miss.” We walked in and showed Miss the phone and he gave it to her to play with. She loved it, and was playing with it so my son went and picked up a toy dog, but when she saw he had it she ran over and snatched it off him saying it was hers. My girlfriend didn’t see, but my son just went and got his toy car out of my handbag and started playing with that so I let it go.
About half an hour passed, and Miss had the phone in her pocket. My son went up to Miss and said “my turn” and she pushed him over and yelled at him “ITS MINE” to which my girlfriend said to my son “well, Miss had it first.”
Son wasn’t hurt, so I just told him to play with something else.
So my son went and started playing with a toy gun that belongs to Miss. About 15 minutes passes of Miss talking on the phone and my son playing with the gun. Miss then starts waving the phone in my sons face and actually said “nerr nerr mine now” (to which girlfriend laughed at and said "haha look shes trying to get his attention"). My son yelled “NO”, didn't even take a step before Miss ran to him then pushed him onto the ground and yelled “ITS MINE”. When the pushing started I stepped in. I said to Miss “we don’t push our friends” and picked up my son. Miss then spotted the gun that he had and tried to take it from him, but my son wouldn’t let go so Miss bit him. My girlfriend finally came over, and took the gun and said “if you two can’t share then no one can play with it” and put it up.
So now, my son is on the floor with no toys and a bite on his hand, and Miss sticks his tongue at him and waves the phone again. That’s when I’d had enough. I said to Miss “Well I’ll have that phone back thanks, if we can’t share it no one can play with it.” Miss then screams at me and says “f*#% you, go away” and my girlfriend says “why do you want to take it off her? They aren’t fighting over that”. I said I didn’t think it was fair that my son had toys taken away from him because Miss didn’t want to share, but she gets to keep his toys when she doesn’t want to share. Girlfriend started to argue with me saying that the gun was one of Miss’ special toys so she’s very attached to it. At this point my son had found his car again, so I didn't see the point in upsetting Miss by taking the phone.

I don’t think this excuses her child’s aggressive behaviour. Normally, it wouldn’t bother me. Everyone parents the way they see fit, and I understand not everyone will agree on how to discipline. But girlfriend is constantly asking me if she can have my son over for a sleepover and I just don’t feel comfortable leaving him there when I don’t trust that he will be treated fairly.
About 6 months ago, I left him there for about 2 hours one afternoon (girlfriend babysat while I had an appointment) and when I picked him up he was covered in scratches and had a bite mark on his back...to be fair, my son bit Miss first that day, but my girlfriend said “you should have seen the look in Miss’ eye before she bit him back. She watched him from across the room then ran and jumped on him and bit him back.” So, in my mind, she basically watched and let her daughter attack my son because he did it first. I think a child being violent is never excusable, but my girlfriend seems to think it's all ok when her child does it and i can't take it anymore.

I don’t know how to say that we don’t feel comfortable letting our son come over without a parent because of what I feel is unfair treatment. I’ve tried the “he’s too young for sleepovers” but I’m hit back with “well, you let him go to his grandmothers.” I feel like she needs to know why, but how do I say it without offending her? I don’t want her to think I’m saying her child is horrible. I don't think she is a horrible kid and my son loves her, but I just can't stand the way he can be treated at times and feel I need to be there to stand up for him. I NEED HELP LADIES!

From one Mama just trying to do the right thing by her son xx