Wait a minute, here. SHE...the MOTHER tossed her own son out! Then she comes crawling to him to try to mooch off of her young adult son who's just trying to get by himself after being tossed out - and now he is carrying the burden of feeling guilty for wanting his mother to leave because she is drunk? I'm sorry, but that's just not fair.
He needs to throw her out, for his own sanity. SHE is the parent, not the other way around. I spent way too many of my years parenting the parents and I can tell you...now in my 30's it breaks you down.
If you want to help him, please support him emotionally and give him the strength to know that this is her problem and he is NOT a bad person for wanting to live a healthy, normal life. Mom needs to go....she can go find somewhere else to destroy herself. Not his job to help her and until she cleans up, nothing he does to try to help her will be acknowledged or appreciated. She'll just use it as a crutch (i.e. free money and an easier way) to keep doing what she's doing.