Medium rare is not raw, it's safe to eat, and prime rib is a really expensive cut of meat. There's no point to cooking it past medium rare, because cheaper cuts of meat taste just as good if you're just going to cook it to medium or higher. Also, "incinerated it beyond all recognition" isn't medium well, and certainly isn't appropriate for an expensive cut of meat.

if someone's cooking for you, you can ask to have a piece more done, but especially a prime rib that's meant to be medium rare you can't make everyone eat it well done.

I said that the mother was wrong to mess with the prime rib. But she's not wrong to want it more well done than medium rare, that's just her preference and it's a totally fine preference to have. Just because you like it one way doesn't mean that everyone has to.

You should never have to cremate good beef. If they want blackened beef get a chuck roast, not someone else’s prime rib. Also, it’s easier to cut the beef medium rare and cook it more for the uncultured heathens that would order prime rib well-done.

Again, Mom was wrong to mess with the food. But she's not wrong to want well done prime rib, what she likes is her prerogative. She just needs to use her words next time.