He wants his mom to move in!

Sooo my boyfriend and I have been together for 10 years. We have a 1yr old daughter and are currently TTC. His mom came to a point where she decided she wanted a brand new car, $600 payments. She lives in an income based apartment by herself and now sees that she can not afford both. My boyfriend has come to me asking if she could possibly move in. I’m on the fence about it because she has already borrowed money from him ($500) last year because she apparently didn’t know that the bank took the money for her car note,so she couldn’t pay rent. I had a major issue with this because rent is always due at the first of the month and she asked him for the rent money on the 14th....since then he hasn’t given her money. I don’t know how to tell him that she needs to figure it out her living situation on her own but at the same time I would want to help my mom BUT I know my mom wouldn’t be in this situation at her age. I don’t want to be selfish or non supportive. What would you do ??