Sounds like she might be a Supertaster. Not uncommon, especially among women. Bitter tastes are stronger, so green veggies are out. Sauces that are the right strength for a normal person are far too strong. A lot of food is probably too salty or sweet, too.
Supertaster or not, it sounds like this is something she should be able to take responsibility for. She's old enough to tell her friends about her needs proactively. She can ask what's for dinner before accepting the invitation, and if it's something she knows she can't eat, decline the invitation - and, perhaps, tell her friends about her food issues so they know to only invite her on days that they're having something she can tolerate.
That doesn't mean you can't help her out, though. In particular, I would see if you could help her learn techniques to cope with it.
For example, many people can't drink coffee without milk - too bitter, right? Milk is a coping mechanism. So is sour cream in spicy food - it dulls the spiciness substantially (as it is a fat).
Help her learn what things she can do to help cope. There may be commonly available toppings, condiments, or other things she can add to foods that she has trouble with that make them more palatable. Bread is a great starter - most people keep bread around, and bread is super bland. Crackers, potatoes, rice, etc.; most plain starches are excellent at making strongly flavored food less strongly flavored.
Teach her how to make simple cream sauces - or even milk sauces (cream sauce substituting milk, since cream is often unavailable). Largely unflavored, except perhaps with cheese added, they are effective at making veggies less bitter. When over at her friend's house, she can ask if she can make it herself - it's not hard to do, and no expensive or uncommon ingredients. She may also find other simple things she can make or even bring to help her tolerate the food.
Consider eating before she goes over - does she have a food budget? Can she go eat at Subway or wherever on the way? Or come home, fix a sandwich, and then go over.
And, finally, have a conversation with her about how to handle things if she's in a situation where she's just not going to like the food. Eat a bite every minute. Engage others in conversation. Help her come up with ways to enjoy her night regardless of food options.