I don't know what I should do?

Me and previous ex broke up a couple of months ago and back while we were dating I knew she had a thing with her bestfriend who happens to be a guy which she knows since ever but she also liked me so I didn't really say anything. Yesterday was his birthday and then afterwards he posted on my FB wall saying "I fu**ed Kate." boasting it, saying he got more action than me with her. I know for a fact its true so just go with me. I felt really betrayed and pissed at the same time. He's lucky he lives an hr away. But is that even right? having sex with a bestfriend of the opposite sex? I mean I have pretty close bestfriends but I would never do that. Whats messed up is this kid is still in 8th grade. I'm tempted to text her and say something about it but we haven't texted since our break up.