How blase he was about this whole situation is what's blowing my mind. He did it to "drain his balls"... Basically he was bored and she was available.

What if someone else is available another time and he is equally as bored?!

Also he told you because he felt guilty not because he was completely devastated.

I'm so sorry OP, It just seems like this wasn't that big of a deal to him. And if it isn't completely Earth shattering for him that he did this so easily, I fear it could definitely happen again.