I find this sort of situation quite strange, to be honest.
Rather severe social ineptness and one or two sort of OCD issues, result in not eating anywhere but home, except for two groups of people - about three times per year in total.
Severe dietary restrictions - and the two mentioned groups respecting it and making provision - has not ever been a problem.
Neither of these two groups are family as close as parents or in-laws.
One would actually expect close family to be rather eager to help, than to thwart. This, however, is said from the perspective of one who has little understanding of these kinds of social issues, expecting the ideal behaviour from people who're supposed to be unequivocally trustworthy and supportive.
These in-laws' conduct has possibly less to do with diet and the meals issue, than a possibly underlying - but effectively suppressed - typical son-in-law and daughter-in-law "resentment". "Summer" has more influence on their son than they have; such a drastic change in his life couldn't've been a simple coffee-or-tee decision.
The answer remains the same: eat according to your choice, whatever it would entail. The point here might be understanding the reason for the in-laws' attitude, which might determine the best course of action - eventually not exclusive to the diet-and-meal issue, probably.
The one answer demanding that one should eat what you're served, unconditionally, is unfeasible. It might be interesting to see the user's reaction when, in the same situation, being informed that the delicious goulash to which he has just sat down at a formal dinner, was made with the best grade dog meat, horse meat, human thigh or whatnot, proving to be served with utterly alien "vegetables", mouldy bread and casu marzu (cheese with live maggots in it).
For the past ten years, the mentioned two groups of people invited me to attend their outdoor functions on occasion, when at the utmost only the members of the other group are present. At first, I've explained to them that I'd attend, but not eat anything. Always in the process of attempting to gain social skills, I've realised it makes them uncomfortable when I don't have anything to eat, and I've started to take my own stuff with. Since they've realized how simple the diet and the solution is, they make provision for the one or two sorts of things I can eat. They want my company, for some reason, and they're more than prepared and willing to accommodate my dietary restrictions.
The point is, whether or not my observation is right, it's rather obvious to me that in the case of "Summer" and her husband, another dynamic than the simple issue of their diet is present. I know more than one close family setup that's almost identical to this, and not once have I heard about parents or in-laws behaving in this manner. The parents or in-laws might consider their children's diet to be a bit out of the ordinary, but I've never heard them being other than absolutely accommodating about it.
On the other hand, I know of families where the "repressed" son-in-law and daughter-in-law "resentment" issue has led to the most bizarre sort of behaviour, even though it might be quite subtle - many years after the children'd been married and the grandchildren already in secondary school.