It sounds like your mom thinks someone sabotaged her meat. If I had any suspicion that these people had sabotaged my meat, they would no long be welcome in my home. And I would say that for anyone I suspected of sabotaging anything in my home. My home is where I get to be safe, if someone can't allow me to be safe in my home, they don't get to be there
No they do not get dictate the contents of another's kitchen. Nor do they get to make comments about it and be considered polite.
But I don't think vegetarian guests get to dictate their hosts not have meat in the refrigerator just because they are staying there.
Guests do not get to dictate the contents of their hosts' fridge, freezer or cupboard. The visiting girl was beyond rude.
OTOH, I can't understand why hosts' would have a problem with guests storing their own (guest) drinks in the fridge.