When you are bulimic, 1/3 of the calories stay in your stomach when you purge. So there is definiely a chance of weight gain for some bulimic's. When they start to eat normal again, with no more binging or purging, it can screw up your motabilism at first, depending on how long you were purging for. You(your friend) should definiely tell a proffessioal about this [doctor/dietician]. I also know that people whith bulimia need a lot of THERAPY to get better.
Please help your friend and proffesional help is what she will need, for the beginning. YOur job is to be there for her whenever she needs you.
Good luck!* I hope this was helpful.