Help! I've seen this question before but answer is always different! Need to know whats true and what isn't!?

Ok,my nephew has lived with my mom for 10 years. He has no job, pays no rent, and constantly breaks the rules he agreed to when he moved in. He makes her life a living hell and she has asked him to leave. He says he doesn't have to leave because he gets his mail at her address. Does that actually give him the right to stay there? Ive seen similar questions but the answers are always different. Some say he has no rights,some say he does. Some say since he doesn't pay rent he isn't a tenant, some say he is a tenant. I need to know how to advise my mom, she is a nervous wreck. She is 80 years old and doesn't leave the house. Will she have to take this all the way to court? We're in California, by the way. I have been researching this for hours but with the answers being different,I'm totally confused. I've tried legal websites, and other forums but I'm not getting anywhere because I can't figure out if he is a tenant or guest. What does him getting his mail at her address have to do with anything%3Help! I've seen this question before but answer is always different! Need to know whats true and what isn't!?
He has no rights at all. Pack his stuff and put it out on the lawn, lock the door and if he fights about it call the cops. His mail has nothing to do with it, I can forward my mail to your address by filling out a form at the post office, no big deal.Help! I've seen this question before but answer is always different! Need to know whats true and what isn't!?
No he has no rights at all. She was good enough to let him stay and now he has not lived up to the agreement, So she has the right to say get out. It sounds as if he is intimidating your Mom, Tell him he can get his mail at the post office, have him open a post office box. I f he won't go, then you will have to go to court.
No that dose not give him the legal wright to live at that address. his mother could have him removed by the courts by declaring that is there as a tenet and that he is not paying his rent at witch point she could have him evicted.