hooking up with best friends ex

I need help!!! Last night I was super blackout and don't remember anything but I hooked up with my best friends ex and I don't know what to do. They broke up two months ago but she is very dramatic about these things. She has been going through some stuff with her eating disorder and is in a program trying to fix it but when I told her about me and her ex she was so hurt that she moved down a level in her program. I feel so bad and I want to help but I don't know how. What should I do?