I have a personal friend that was a co-worker for 10 years that now reports to me at work. HELP!

We were friends for almost 10 years, and worked together multiple times. The scenario presented itself to work for me, and get out of an uncomfortable position he was currently working in. Now I am the bad guy, he has resentment for me because I am a manager and he is not, and sends emails to me saying "Between friends, I am not being treated fairly" etc and rails against me. He believes because we are friends that I will not report him and that he can speak to me "offline" or "work aside" in this manner. I don't want to betray our friendship, but I am tired of the disrespect and the stress of this is draining. Any suggestions?