Should she leave? 🤔

I'm usually great when it comes to giving advice but today my sister asked me if she should stay with her guy friend. They started talking last April (2017) in the beginning he explained his living situation (he stays with his child's mother) but it's just until he gets on his feet. It is now February (2018) they have been serious now for I'll say 5 months now but she doesn't really like that he stays with his child's mother she'll be moving into her our place next month. She said they are both honest with each other and he says he's not having sex with his child's mother. She wants to know what to do SHOULD SHE LEAVE HIM ALONE... SHOULD SHE STAY A LITTLE BIT LONGER?? I'm actually stuck and could really use some help.

p.s. he has 3 children (girls 10,8,& 9 months) he stays with the 10 year old daughter...my sister has one child a 4 year old boy