Recovering from an eating disorder, whatever the severity of it is, is extremely difficult. It takes so much hard work and your girllfriend has done to even go to the group sessions. You just have to understand that she's trying to give up a habit, a way of life and though it seems terrible that she binged right near the end of finishing she is just relapsing. Don't take it as her giving up on all her hard work, she just had a slight lapse in her recovery.

If you want to help her try and ask her to explain her feelings about it. Why she thought she did it and stuff. It could  be a sign that she needs a little more help to recover. But the best thng you can do is let her know you'll support her through it all.