Similar thing happened to me. My friend and I were, and both still are, into the same girl. Neither of us knew at the start and she didn't know that we were friends so she never said anything. Two weekends ago I go hang out at her place, meet her parents, stay for dinner and end up fooling around. Thing is, she only told me then that he also likes her. So now I'm stuck in a situation where he can't know but she wants to hang out every weekend. Shit is tough as fuck to play out especially when I feel super bad for my friend, who still thinks he has a chance with her. Fuckit.
You're in the clear. Be honest with your friend that you had no idea that he was pursuing her and that you've already won.