Ever dated a girl with an eating disorder?

I recently discovered my SO has bulimia. She admitted to me one night that she sometimes throws up after eating to avoid looking bloated, but she said she rarely does it anymore and that it's no longer a problem. I know that's not the case because ever since she told me that all the signs have become obvious. She can eat more than any girl her size that I've seen while still keeping a great body. She always goes to the bathroom after eating or sometimes drinking. Sometimes I can hear her vomiting in the bathroom and when I ask her about it she denies it and says she doesn't want to talk about it.

tl;dr: SO has bulimia and doesn't like to discuss it, what sort of action should I be taking?