I have suffered from bulimia for the last couple of years. For me, it got triggered by upsetting events (eg. my bf breaking up with me), and would go on for a few months, then I'd think I'd got over it and suddenly something else would happen and it would start all over again.
Just so you know about some warning signs, I got scars on the back of the knuckle of my middle finger, headed straight to the bathroom after meals, and my weight fluctuated a LOT (I would put on/lose 10kg in a month).
I've never used laxatives, but that is definitely a bad sign, as is the depression. Does she eat a lot in one go/eat in secret? Have you noticed a lot of food disappearing/wrappers in the bin? Bulimia includes binging as well as purging - if she doesn't have binges then it could be some other kind of eating disorder.
Just so you know, I would never want to admit it to anyone, let alone to a bf that I wanted to love/respect me. If you do bring this up with her (and she is bulimic), she'll probably be really embarrassed and ashamed, so you'll have to be reallyyy sensitive. Once you get past that, she'll probably be grateful that she has someone to support her, and you should make sure that she gets help from a doctor, because bulimia is really dangerous for your health in the long term.