This logic is wrong. Bulimia MAKES you fat. It not only puffs up your face, but it makes binging habitual so you have eat more to feel full, then you gain weight from that or have to purge. I would suggest she gets rid of any "trigger" foods in her house as a first step.There's also lots of places she could do inpatient treatment, but if she goes the route, make sure she does her research.
you should try to support her as much as you can, I'm talking from experience, because I also suffer from bulimia and I can honestly tell you that sometimes a person needs someone who understands what they're going through, and be there for her as much as you can, and maybe try and tell her to talk to someone about it, like a doctor or a family member