Aren't there some report/block functions? I think you can block certain users to contact you by messages.
But I am not sure on this, either way, ignore the guy, or better - save the mails and show them to the police if they are real offending. Then the bastard will know his place.
Your a good friend that sticks up for your friend, and you should not need to take bullshit from a other person just because you defend your friend.
Either way, that is none of my buisness, but as said, if it's treatning, save all the messages he sends, and then show them to high authorities.