I know this isn’t what you want to hear, but there’s not a lot you can do to help her aside from being there for her. Of course you can help her become more body positive, but you can’t make her, you know?
That being said, one of the best things you can do is make sure you understand what you can about bulimia, including the myths and stuff surrounding it. Also, if you know what triggered it, you might be able to help her overcome it (although you definitely need to be careful, because it could make it worse bringing up these memories. A professional would be best for this).
You can also encourage her to get help. Whether it’s talking to a school counselor or (preferably) a therapist with expertise in the field, explaining her feelings might be cathartic for her and help her overcome her eating disorder.
This is a website that has a lot of information about EDs and talking to people about them.
A final note: I know you want to help her, but you can’t go in with the idea that you’re going to fix her, okay? She doesn’t need you to fix her, but she probably needs a friend to help her start to help herself. It’s also not something that’ll work itself out overnight. It’s gonna take work.