I've been looking at the responses and it looks like men and women really deal with this differently.
My two cents worth is, to talk to your friend and find ut what kind of relationship they really have, (if she will tell you).
Some people don't find "the one" when they find a partner, and make arrangements among themsleves to "be with each other on conditions."
Relationship are complicated and private to the people concerned. He may have told her that he is still on other sites.
JMO but it's not your place to burst anyone's bubble just because you saw his profile. I have profiles on several sites and go visit them to see if there is mail from time to time and I am in a relationship. It means nothing.
If you had definite proof that he had made arrangements to date someone, then that is different, but there is no proof of anything here.
It is all too easy to jump in with our own agenda sometimes when theirs might be a lot different......