They have had an open/poly relationship for over a year, he's openly about it several times.
Yesterday night, I saw him at a party. I casually asked how their relationship was going now. He said they're still completely open, but not certain on the poly part.
Fast forward a few hours and a few more drinks, we kissed. When I came back from the bathroom, he pulled me into an empty room, closed the door and put his hand in my pants. We hooked up. He definitely initiated everything.
Today he called me to tell me he cheated, what happened yesterday wasn't okay and his girlfriend dumped him. He apologized to me for involving me like that.
I'm very confused now. I feel a bit bad, although I also feel like I couldn't honestly have known and I didn't start anything. I'm disappointed in him too, I expected better from him tbh.
Is there anything I can do to help the situation besides staying a long way away?