its ok. i wouldn't like it either.
if he only talks with her every blue moon then i'd just suck it up and deal with it. its rare, but if hes still hanging out with that old group of friends and shes apart of them then i'd make it a point to be invited and have those friends like me too. THAT WAY if there is something to worry about you'll be aware of it. ^_^ make friends with her. she won't try anything with someone she really feels like shes friends with. and if shes distant then tell your guy that you REALLY don't like it and it hurts you that hes spending so much time with a girl he had feelings for.