cheating and friend's loyalty

I "dated" or went out with for like 3 month (nothing serious) my best friend's friend because we really liked each other (she was never my girlfriend but we both told each other about our feeligs), but in the end she cheated on me.

When my best friend found out he really stood on my side, even insulting and telling her mean things (which she deserved anyway). I've never spoken to her again ever since, but now they both talk from time to time.

When I think of it I usually get angry because I think my friend should do the same as me and ignore the girl (Because it wasn't even a long time friendship, I went out with the girl about 8 month after they met ).

I don't remember if I told my friend that it was ok for him to talk to her, but I want to know if I'm right about it (and my friend should forget about her) or if he has the right to talk to whoever he wants.