In California. How do I assist widowed mother in evicting youngest son. He will not work, has no working car, but always has money for beer and pot. He says he will live in a tent in front yard if Mom throws him out.

First of all is he 18 or older? If so than she can just kick him out and as far as camping on the front lawn goes she can get him in trouble for a nunber of things from trespassing to urban camping.
My suggestion is to tell him to kick rocks.

A lot is going to depend on where you live and what's going on. If he has no job yet " seems " to have money for his habits you have to ask yourself.. is he dealing drugs??
If he is then Im sure the local police will assist you in his removal pronto. Remember through all this your mother may have objections and sometimes when a person gets along in years and loses someone they can get lonely and it can open the door for abuse. You might need to help her with this and maybe even get counciling or therapy for her or both of you. Let us know how it turns out.