I actually went through a similar situation a few years ago with my mother in law. The guy wasn't alcoholic though, but he had mental issues. He didn't work because of it.
They came to visit once and at like midnight he was downstairs making noise so we told him to be quiet, and he started yelling and screaming, so we told him to leave. Her mom sided with him for quite some time afterwards, but we knew the guy and his character. He sort of brought her down to his level. She stopped working and they sat around the house and got fat. We wouldn't go to her house when he was there, and he wasn't welcomed at ours. Eventually, she saw the same things we did and got rid of him.
I think the problem with, "putting aside differences for your mom," is that he can pretend it's ok. If you disapprove then you disapprove. No sense being fake. You can still be nice to your mom - invite her over without him. Go places without him. You don't need to pretend the guy is ok. If he wants to show for a long time that he's a square guy then you change your opinion then.