Should I get even?

[Jeni]

I would say that you should do nothing. Show this ex-friend of yours that you are the better person and that a little petty betrayal like this won't send you over the edge. Show him that you are the stronger and more mature person. I know it's hard, trust me on that, I've had my fair share of betraying friends.... but what he wants is to see you get mad and do something stupid to prove himself better to this girl. But if you act mature about it, she might just think that he is just a child and that what he did was immature!
good luck!