She has a car and drives to see them often. We don't want him in our home because of past drug abuse and various other reasons. We had her living here a couple years ago and he talked her in to moving in down the street from him talked her in to selling her car, said he would get her to work, store ,church, etc, then got in a big argument and he wrote her off even cutting his young children from her. She has since moved back in with us and got a car, still works 2 days a month. She has a hard time getting around because of a back injury, that's why she needs to live with someone .
If she is a tenant, even without a lease - meaning, she has established tenancy under the criteria in your state, then she can invite him over as her guest, even though it is your home. BUT, you can make her tenancy conditional, You can make the rule that she cannot have certain people over or she can forfeit her right to live with you. (not a lawyer but spent a long time in law enforcement and a bit of time in landlord tenant court)
Morally and ethically, I feel that as long as she has a way to see him, you do not have to let him into your home. You can have them see each other at his place or a restaurant. If she were to become homebound, I think I would allow him opportunities to visit but would supervise closely.