How to handle awkward conversation please?

[SunnySomer]

Brief background: we moved house last year and 9YO DS changed schools. He is happy at the new school, seems to chat to a decent range of people in his class, has got involved in a number of activities outside school, keeps busy - all OK.
However, he doesn't seem to be making good friendships. He's become default friends with the other child in the class who was new on the same day; this is kind of ok, he's pleasant enough, except that the more I've got to know him, the more uncomfortable I am with DS spending too much time with him. Principal reason being that he goes to bed at midnight (this not 9yo bragging - his mother told me so), and because his mother is "busy" in the evenings (working and TV - again, she told me this) he has unsupervised access to the Internet from end of school until going to sleep. He mainly goes into minecraft and similar game chat rooms, as far as I'm aware. She has told me about some incidents online involving her son that I find horrific and she laughs off 😨. He is quite naive and trusting and, I think, really quite vulnerable.
The boy's father (parents don't live together) keeps bombarding me with texts and emails asking for my DS to go there for tea, go for the evening, go for a sleepover, go away with them for a weekend over half term and all I think is "noooooooooo". (Quite apart from this DS is not keen on the father and refuses to go to his house anyway).
So far I've managed for us to be almost always busy, but I actually think I need to say to them that I'm not comfortable for my DS to spend time at their houses because of the unsupervised internet access. Can anyone advise on the best way to do this without basically saying to them that their parenting is inadequate?

[OnTheRise]

Easy. You say, "Thanks for inviting my son over to yours. I'm not comfortable for my DS to spend time at your houses because of the unsupervised internet access and very late nights, so it's not going to happen, but he's welcome at ours this weekend."

It's better to be straightforward, I think, than to dance around it. You're not telling them their parenting is inadequate, just that you don't want your son treated that way. It's different.

[SunnySomer]

Thank you. I'm sure you're right. He does already come to ours a lot! Right. Time to muster up my courage!

[EyeoftheStorm]

Agree with OnTheRise. You just need to be straight. The relief will be enormous after all the dancing around.

Been in the same situation a couple of times and both times I was honest in as kind a way I could be and everything worked out fine.