re: What's the statute of limitations on when it's OK to screw your buddy's Ex?

Maybe never. Depends on your friend, the type of relationship she had with him, the type of relationship he has with you, his temperament, etc. I think you have to be willing to accept that you are possibly ruining your friendship, generally speaking. If it's worth it to you then you probably aren't particularly close friends with this guy. Or maybe you're serious enough about her to abandon your friend.