yeah, asking first is a big part of it...
I wouldent be jumping for joy if a mate was dateing an ex of mine... but id wish them luck as long as they thought theyd be serious...
EDIT: tho i guess how it ended with the girl in the first place, thinking more about it... it would be ok for a mate to go out with most of my exs, but theres at leat one where id feckin kill him !