Boyfriend kissed a girl he says was threatening to kill herself, both were drinking. What would you do?

My boyfriend is my bestfriend. He always said we were so strong because of our complete honesty, open communication, and trust. He also knows of my past trauma with a guy who cheated on me with a friend and said he would never do anything to ever hurt me in that regard. Fast forward to this summer, he got a scholarship to study a language abroad. Long distance was seemingly fine- talking everyday, always landing on a good note. One night last week he tells me he's not feeling so great and goes to bed early. I don't hear from him until the following night. Out of nowhere he talks about how stressed he is and that he needs a 3 week break (until the end of the program). I find it weird but apologize and agree. The next day, I push him for the real reason, he again blames it on being overwhelmed with the program.

FINALLY he gets me on the phone to discuss all these underlying problems with communication he feels we have that he never voiced (feels he can't tell me everything) and admits he went out drinking with 2 girls- one is his good friend and another was some "crazy suicidal girl with a drinking problem." She gets wasted, puking in the bar, they get kicked out. She's screaming the whole way about wanting to kill herself and tells him to just take a knife to her throat. They end up on her apartment staircase. She tries to kiss him, he pushes her away. She tries a second time, he kisses her back, then takes her to her apartment and drops her off.

I want to believe he really was scared and felt bad for her and thought kissing her would make her stop screaming (his story). He says he isn't attracted at all and can't understand why he would let this happen when he "loves" me and our relationship and knows my no 2nd chances stance on cheating. But I can't get over the choices he made that lead to that situation, the fact that he needs an emotional connection before ever being physical, and that his instinct was to hide/ lie to me. What would you guys do?
Boyfriend kissed a girl he says was threatening to kill herself, both were drinking. What would you do?
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