I slept with my best friend's girlfriend

I was staying at her place. We were both rather down; she felt bad for me that my g/f dumped me and I felt bad for her that she had a bad moment with her boyfriend. She offered to sleep together since she had no spare beds for me. Then it happened. Shit, shit, shit. What do I do? Do I tell him it happened? I don't feel particularly good about what happened. Shit.

Well they broke up. She says she didn't tell him. I feel like I broke their relationship apart.