Am I the only one who would at least want to see the Dad since I and my DD liked the little girl and the Mom so much? I would want to know what is going on there before I write them off. Good friends can be hard to come by, and it sounds like this was a natural friendship. (And I also got a strange vibe from the OP, but I'll play along anyway.) I also understand the value of having some exposure to normalcy for a child who really is stuck in a bad situation. Would I let my child go over there? Likely not if I saw bad things and also met a person I didn't care for. But I would probably still let them play at the park and have the girl to our house, if it worked out. A child shouldn't have to pay for their parents' sins, and we, as parents, don't have to be best friends as adults when our kids are friends. I
would be concerned with unsecure guns and seedy people being around my DD over there if it really was what it appeared to be. But this girl could be the sweetest kid and just stuck in a bad situation.
My DD actually had a friend sort of like that. Mom let her stay overnight at our house one time without even meeting us, just dropped her off at the doorway, and left her ALL the next day without checking in while she was off with a guy somewhere; picked her up after dark. I felt sort of bad for the girl. Fast forward 15 yrs, DD ran into her somewhere recently and they hugged, and this girl excitedly suggested they get together for lunch soon. So DD texted her later, and the girl said she couldn't go because she had a class, but DD saw her posting on social media she was actually with another friend out at a restaurant. So DD wrote her off then, because she lied - but not because of her family situation.
Growing up in a difficult family situation myself I'm glad many of my friends parents always welcomed me with open arms because they'd taken the time to get to know me and knew that even as a child, I had integrity and a moral compass and they liked their kids being around me. I had one friend whose single mother worked evening shift and I was one of only two friends who was allowed in their house while she wasn't home because she trusted me. I realize times have changed with raising our kids, but I'm almost always going to give a kid the benefit of the doubt unless and until they prove themselves unworthy. That also happened to one kid we knew who was a nice kid, I thought, and DD was contemplating going out with him in HS. They were texting, and out of the blue he said some highly inappropriate things to DD. She was hurt, and he tried to say it was his friends who stole his phone and sent the texts, but she didn't feel she could trust him anymore, and that was the end of that. Kids learn from these situations and with good parental support there's no reason they can't begin to navigate them even at a young age. It sounds like these two from the OP might be classmates, will OP's DD learn to just ignore or stay away from this other child?