Depends on the people. I'm still good friends with my Ex twice removed. My recent Ex, was a little jealous of her but I didn't consider that my problem. I'd like to stay friends with my recent Ex, although I'm with someone new, but it doesn't seem she's interested. Anyway, more to the point, assuming they aren't fooling around, they have every right to be friends. It's up to you to decide if you can handle that or not.
"Oh Lord it's hard to be humble, when you're perfect in every way. I can't wait to look in the mirror, cause I get better loking each day. To know me is to love me, I must be a hell of a man. Oh Lord it's hard to be humble, but I'm doing the best that I can." Mac Davis