mil drama 😕 just stuck in between a rock and hard place

For the last 5 years I've been telling her to move and she simply refuses to downsize. With a 4 bedroom house she only shares with her eldest son, that's not working right now, and the house is a disaster because she ducks her landlord. 

Now yet again she's late on her rent. She calls my husband because we are the only ones besides the son living with her that have no children. I understand everyone needs help sometimes but he's giving his mom 3-600 a month to float her by.. the last two months he hasn't because of our wedding costs and he was laid off work for a week but doesn't this just show she can't afford her living situation? Oh and now he's going to pull from his vacation check to make sure she gets ahead and handles this months rent.

Idk what else to do because he knows how I feel about it and I always tell her she needs to move