if the friend that betrayed me was a boy:
i'd get back at him and probably take all his friends away from him and make him regret

(yes i could be pretty cold hearted when it comes to fellow guys

)
if the friend that betrayed me was a girl:
i'd probably ask her why she did it, ignore her for a couple of days or weeks then if she regrets what she did i'll make her my friend again

see how i treat a dude and a dudette unequally? lol