You both decided that sleeping with this guy was more important than your friendship. The friendship is therefore over. You've both made it clear through your choices that you don't value the other person enough to consider how your actions impact them. That's not inherently bad or wrong, per say, but it makes both of you pretty shitty friends to each other. In other words, you weren't acting like friends to each other, you both realized that, and now you are no longer friends. You won't be able to ever trust each other again or respect each other and those things (trust and mutual respect) need to be in place for a friendship to exist. If you only wanted to sleep with this guy once and knew your friend wanted to also sleep with him and had strong feelings about you doing so then the choice to sleep with him was a choice to destroy that friendship. It sounds like you made that choice, and then so did she and you both compounded things by failing to be transparent and upfront about the fact that you did make that choice. It sounds like you both acted in inconsiderate and destructive (to the friendship) ways. Move on and recognize that you both acted poorly by not realizing that a one night stand with this guy probably wasn't worth the drama or worth potentially hurting someone you claimed to be friends with. Or maybe it was? Either way, this is one of those situations where you've made your bed and all you can do is walked away from it. Either you both have the maturity to forgive and forget and move on (which it doesn't sound like you do based on the ways you've both acted this far) or it's time to call and end to the friendship.