Advice on dealing with mentally unstable tenant in CA

[John Chang]

Hi, I just received a phone call from one of my tenants in a duplex that she had to call the cops on the other tenant in the duplex because he was up all night banging on the walls and screaming. She said that he has had this behavior before, for a long time, and sometimes he will go into their backyard and pull out wires and stuff and she said that she is scared because she knows he has guns. She said she thinks he's mentally unstable. 

The day after the cops were called, he called me (before I found out about this from the other tenant) and he said that he was very sick and had to go to the emergency room the night before. I wasn't sure why he was telling me this, but now I figure its probably to see if I had heard about this incident from the other tenant. 

This is the first time I am hearing of this behavior and every time in the past I have talked to him, he seemed fine, but I have not talked to him extensively. How should I bring this behavior up to him to keep the peace between the both of them. I don't want my tenants to be scared. I have thought about evicting him because he never pays his rent on time anyways, but if he is mentally unstable, I know there are laws in CA protecting his rights, so I'm not sure how to go about this. Any help? Thanks