So in the example I gave, my DIL was at work and there were scones and jam put out.
She was given a gluten free section and told the knife was clean. It obviously wasn't clean enough.
To clarify:
I'm lactose intolerant. I think the hosts should cater for people who are intolerant to either lactose or gluten. I'm relatively easy to cater for as I just need to not eat it and I can tolerate small quantities.
Even then it's hard for other people as lactose is in a lot of stuff that you wouldn't necessarily think.
My mil used to make lasagne with cheese and then just leave cheese off a section at the end. The problem was the cheese would spread.....
I would expect his family to cater for him. We make all the meals gluten free when my daughter in law comes. Nonetheless, I've been tripped up by things I wasn't aware of - soy sauce, malt vinegar etc.
I now check labels very carefully.
She only eats at coeliac certified restaurants as the ones that have done food gluten free do tend to have some cross contamination.
I've been told that water and normal cleaning (so washing up liquid) is good enough to prevent cross contamination - and on plates that seems to be the case but it's so easy for a crumb to be left on a culander or sieve or edge of a knife.
As a person making the meal, the only way to be really sure is to use essentially a completely different set of pans, knives, etc that have never been used for gluten. That's expensive and hard to do. I mostly do that now.
So the only way he can be really really sure is to take his own food. I'm now divorced and I don't see mil anymore but that's what I did.