I'd be inclined towards the following response
"BB, I'm not interested in hearing this. I am not interested in you. We are both married, and telling me this isn't fair to either of our spouses. Please do not contact me again."
If it's him emailing you, he's being an idiot (I'd use stronger words in person). The time to confess feelings for a friend is before either of you are married. Once you are both married, your feelings become a deep dark secret, shared with no-one. If you are married to someone else, and still carry a flame for an old friend, then the fair thing to do, with respect to your spouse, is to cut off contact with the old friend.
If, by some chance, his wife has sent the message to test him (and you), I'd sent the same response. If it's his Myspace account, he will be able to see the message and your response, and can deal with it accordingly.
In that case, if he's chosen to stay with a jealous, manipulative, emotionally unstable women who is deliberately sabotaging his friendships, then this is what he signed up for. If he's married to someone with that attitude, then he has to accept the fact that he will not be able to have female friends, old or otherwise.