Anyone seen Forks Over Knives? My MIL is all about it, making everyone watch it, upset if they won't, etc. From my bit of reading the science isn't great and some of the things they claim just aren't backed up by the studies they cite (China study mostly) and they use a lot of animal studies (not exactly analogous).
Movie wasn't bad, and I don't think anyone argues that tons of red meat is healthy, but I'm not sure I can get on board with the completely vegan for health reasons. You do you for ethical, environmental, whatever reasons though. I like my fish and chicken and occasional steak though.
If she was less sanctimonious I wouldn't care. Don't get me started on gluten. Whatever the flavor of the month is, she's on board.