I was reading this thread on MeFi https://www.metafilter.com/182435/Toxic-In-Laws and man, yikes.
There is a weird under-current of trying to stake out the line between legitimate dietary needs and one's the host is free to dismiss and oof.
I've always thought that however it is someone arrives at their dietary restrictions, it isn't up to me to decide whether or not that's "real" 😦