what to do with his gun?
You say that he is your friend. So why do you feel that you have to sell it or give it away? What are you going to tell him when he comes by and finally asks for his gun? Most people on this site, like multiple guns and are always looking to add more from time to time. Just keep in safe place and go shoot it from time to time. Nothing too wrong with an extra gun in the house.
If you sell his gun, someday you will probably have to explain why you did that.
Final soulution: If you feel you have to get rid of it, send it to me and I'll be happy to put it in my safe for "safe" keeping....