I don't think the OP said she is currently cooking for her FIL did she? Only that he could form a bubble with her family and she would be able to cook for him? May be wrong.
Personally I would stop worrying, just let him do what he wants but doesn't mean you have to put yourself at risk by going to see him either.
I think everyone who has capacity should know exactly what they should be doing to reduce the risk of passing it on. Not in favour of telling others what they should or should not be doing. At 76 maybe he just wants to live his life as he wants? If he and his friends have been isolating for past 3 months, highly unlikely they will pass anything on but up to them to judge. Hopefully, wherever they go to they will observe the distancing rules?
Do you think you know when you are going to die?
