My son's best friend... What would you do?

Lucian was at a friend’s house, today. They’re Polish immigrants and they’re not very friendly toward me. I understand a good amount of Polish, so I know what they say about me that they don’t intend for me to hear.

This friend is six. She has three siblings who are 9, 13 and 16.

I usually don’t leave him alone at his friend’s houses, but my love has finally convinced me that I’m being overprotective, so I left him there at about three and picked him up at six. On the drive home, he looked at me and asked,

“Have we ever had sex?”

I paused for a moment, in attempts to keep my disgust out of my voice, then responded,

“No. Whoever gave you that idea?”

“Valeska (his friend’s sixteen year-old sister) said you’re gay, so you probably have sex with me.”

Lucian, incidentally, has no idea what sex is and hasn’t asked, so I truly don’t know how to respond to the mention of it.

This girl and her family are scum. I cannot stand them. They constantly talk about me and whenever I tell them something that Lucian’s friend has done, that I was not pleased with (She frequently throws things at my cat, despite that even Lucian begs her not to) they act as if it is my fault. Unfortunately, this girl is the only friend Lucian really has and I don’t want to stop allowing him to see her.

I don’t even think this is worth bringing up with their parents, because I truly doubt they’ll do anything about it and yet, I obviously cannot allow him to go over to her house, again if this is the kind of thing he’s going to be hearing. I don’t like doing damage control every time he comes home from there and I can’t very well forbid him from seeing his friend, but all three of her siblings like to tell him some very disturbing things, such as “Your dad likes butts,” and they frequently talk about putting firecrackers in various orifices of small animals.

We’ve established that he won’t be going over there again, but what do I do about his best friend?

Seeing as she lives in a house full of people like this, should I allow him to continue seeing her? Obviously, he can’t go to her house, but should I even allow her to come to mine? I can’t imagine that she doesn’t talk like that if all three of her siblings do, but I’m constantly in the room with them when she’s over and I’ve never heard her say anything too terrible, so I’m at a loss as to what I should do about her.