T.H.
It's not that he's talking to a woman or an old friend, but that he refered to her as "sexy". Not okay, especially if he's being secretive about it. If he were just chatting with her or even if he just said that her new hair cut looked nice, then okay, maybe that's okay, but sexy should not be used for anyone besides his wife...!
If you feel like you have reasons, this and others, to be concerned then you probably do. Confront him, get couseling, shut down FB, ...do whatever you both can do to make it better. I think the problem with FB is that people can be somewhat anonymous and it's "safer" to have these types of conversations b/c it's "not real"...but the truth is having these types of interactions will only lead to more so if you're worried now, talk to him about it.