As a parent of older teenagers, I do have friends that are friends of my kids. I also know them though volunteering at the activity my kids are involved in. I have never sent a friend request to any kid and never will, only responding to requests. Me interactions on FB have generally been punny or I hope amusing responses to the occasional post that just screams for it: IE (What kind of reptile should I get as a pet? -> Nonpoisonous!)
I assume that I am put on an "ADULT" restricted list, and am fine with that. My reasoning for my FB stance is so that I don't get looked at like the FIL is. As an adult male, I think it is on us to be ultra vigilant to perceptions. Interactions with young people not only have to always be above board, but also have to appear that way to others.
I know that the general advice has been to let kids and their parents handle who has access to their social media, but I think it would be a kindness to the FIL to have his son explain about how things are perceived. So if I were the OP, I would let her husband handle it.