This is probably true. Older retired ladies whose lives have become very small (or perhaps always were) do love a bit of drama.
What pps don't understand is that extended family is very important in France, and this is often expressed by sharing meals, as well as adult children pitching in to do little jobs for their parents. In the OP's case it's not the sandwiches or dinner that's the problem, but the fact that the MIL holds such an exalted position in her son's life that he can't seem to stand up to her even when it causes problems in his relationship. I am 100% sure he will not move far away, and I think it's possible that he won't ever marry or have kids; what DIL would put up with this situation?
When I was OLD in France a couple of years ago, I had a few guys cancel first dates at the last minute. What do you think their go-to excuse was? "I can't make it tonight, I've just remembered I'm having dinner with my parents". A British guy would have said "Sorry, I can't make it tonight, something's come up at work". Basically, parents in France are what work is in the UK: the overwhelmingly important priority which it's completely acceptable to cancel plans for.
Obviously, not all families are enmeshed - a lot of people have to move far away for work, for example - and not all MILs undermine their DILs, but it's a risk you take. Speaking of risk, I think it's possible that the OP's MIL would make a lovely mamie, but I'm not sure the OP should take that chance. Basically, she has a DP problem and should find someone who has her back.