I guess I'm looking at this more objectively now because for all practical purposes, I'm an unattached Man.
I have many female "friends" and even as a single Man with nothing to hide, loose, or feel guilty about, I can recognize the boundaries and master my emotions. I can freely cross that boundary between "friend & lover" if I choose to. Regardless of my sense of attraction, I'm still in control of where the relationship goes or doesn't go. The belief that Men are somehow out of control sexually and lead around by their "little head" is BS. Friendship doesn't go magically and uncontrollably from one realm to another unless he so chooses.
Any Man that tells you he didn't see it coming or didn't expect it is lying to you. MLC or not, a Man can recognize those boundaries and know when he wants to cross them.
I would still venture to say that if he can tell his W about his female friend, then most likely he doesn't have intent to take it any further. However, if he starts to go silent about his female friend all of the sudden, I'd beware.
I still think Men no matter what their frame of mind (MLC or not) work off of intent. If he intends on having an affair, he's not going to reveal it (directly or indirectly) unless he has some other ulterior motive.
If the motive is pure unadulterated attraction, then your most likely not going to know this OW without doing some digging yourself. Consider yourself lucky if he exposes her, because he's trying to tell you something and at least your in the fight.
"It is curious that physical courage should be so common in the world and moral courage so rare." -Mark Twain