Why doesn't he tell his mother she is the problem?

We have been together for 4 years and have 2 kids. His mom lives with us because she is the most irresponsible 51 year old woman I know. Her bedroom is a horrible mess that she refuses to clean (food, trash, clothes everywhere) She wouldn't even think about lending a hand to clean the house. She doesn't pay her share of the bills nor does she help buy household necessities. WE buy EVERYTHING not to mention we have to support our children as well. This leaves us with no money to ever go do anything together (like go on a date). She is a mean, selfish b*tch. If he doesn't run when she yells (FROM HER BEDROOM ACROSS THE HOUSE) she gets so mad at him and calls him names. She also calls him names when he refuses to lend her money for bingo, which is where her entire paycheck usually goes anyway. She has treated him like this his whole life. She has 3 other kids and doesn't treat them that way, but none of them will let her live with them. He is afraid to tell her how he really feels because she has a way of hurting him to the core. He has never been afraid to tell me how he feels about her. It's like she dangles her love and respect over his head just high enough he can never quite reach it. How do we get rid of her? He told her she needed to find her own place and she put him on a guilt trip by saying she will live in her van. How can she treat him this way, I could never imagine doing this to my kids. She tells me all the time that I am his problem, but I know it's not really me, he just can't express himself to her so alot of timesa he takes it out on me. I am at my wits end!!!