Stepdaughter's food phobia?

Hi everyone,
I'm a mother to a 2 year old (and one on the way), and have a stepdaughter from my husband's previous relationship who is almost 7 years old. We only get to have her Friday to Sunday every fortnight. I've always been concerned about her eating problems and they seem to have only gotten worse over time. It's not just what she will or won't eat, it's her actual behaviour about food that has me concerned the most, it almost looks like an eating disorder to me. She refuses to eat anything except Maccas nuggets and chips, sausages, and Nutella sandwiches, occasionally she will have lasagne but sometimes makes excuses not to (yes that is all she will eat, literally). Her mother just gives her whatever she asks for, even if it's not good for her health (she has bad constipation problems as well).
We have tried encouraging her to try different foods, and have now put our foot down about no Maccas, but she starts crying if she's told to try a food on her plate. My biggest concern is how she behaves when she sees food. She will look at it, and her face will change like she's horrified by it. If we make her try a tiny bit, she will make a face and move back like she's afraid of the food, then take a tiny lick and start to dry reach as if she's vomiting (when the lick she took has barely touched her mouth). She even once vomited when she took a minute lick of cream from a dessert at a restaurant.
I'm concerned this is more than just fussiness, because it's like a phobia when you watch her behaviour around food. I have mentioned to my husband that I think she should be taken to a professional to investigate where this fear-like behaviour comes from but I think he's scared to enter into a disagreement with his ex over it. His ex isn't exactly easy to talk to, even if it's about the wellbeing of her daughter. Sometimes I feel like I care more than she does. Furthermore, our 2 year old is seeing this behaviour and may start imitating it, as she already imitates everything big sister does. I've got my 2 year old eating a wide variety of food because I believe in establishing healthy food habits from a young age, so I'm also worried it will all change once she starts understanding more what my stepdaughter is doing with food.
What should I do? I feel powerless.