Seeking advice

I absolutely hate my boyfriends mom but he is very close to her even though she is not a good role model, person, or parent. She is completely overbearing and desperately seeks my boyfriends attention she even lays on top of him and cuddles him and constantly uses him as a shoulder to cry on when she is married and has friends. How do I deal with this? Me and my boyfriend have been together for two years and I know he is the one I want to marry so I'll do anything to keep him but when I try to talk about his mom in any kind of a negative light he never fails to defend her and disagree with me it feels like she is coming between us

She is addicted to painkillers and her husband is as well. All of her kids except my boyfriend left her and the two youngest have been living with one of her friends for almost six months now. They hate her because she lies and leaves and does pills.