Don't tell Emma!
I mean, unless your friend wants the awkwardness factor ratcheted up by a factor of a thousand (or at least a little), wants to put Emma in the position of having to rethink their friendship, and wants to risk ending his friendship with her. If so, then blab away.
I would not want to be friends with someone who put this on me, honestly. I'm married, so it'd be all the more awkward, but even before, it would have been a waste of my friend's energy because I'm not the type to go, "Oh, really, you might have feelings for me? I guess I better go break up with my perfectly decent boyfriend just in case you actually do have those feelings, since I might have feelings for you, too, if I dump this guy to give you a shot." (It'd probably be more like, "Oh, really, you might have feelings for me? Good thing relationships are a two-way street, and I'm already in one with someone else!")
His time is better spent refocusing--on other friendships, on dating other people, etc.