Boyfriend slept with my best friend

So a while back, my boyfriend of a year slept with my best friend. They were both struggling with suicidal thoughts, so they spent the night at his house making sure neither of them hurt themselves. I was supposed to go with them, but instead I ended up going out with my guy friend (who my boyfriend isn't particularly fond of). That night, they both got REALLY REALLY drunk, and I spent the night on the phone with them. After I fell asleep, I guess they slept together. 
However, it only lasted a few minutes before they pulled out and stopped bc they realized what they were doing. 
Afterword, they told me in person, apologized whole heartedly (both of them cried a little and my friend even offered to let me hit her). He then spent the entire night trying to make it up to me because he felt so bad. Even though I really didn't react to it. 
Keeping in mind, before this, I had told my boyfriend that if he ever cheated on me and it was an absolute one time thing, he didn't have to tell me bc I figured that keeping a happy relationship after a one time mistake would be better than ruining a happy relationship with a one time mistake. So in theory, he really don't have to tell me, but he did anyone. 
So in conclusion, would you be mad or would you let it go?

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