I can't speak for cats, but it's certainly possible for dogs to do fine on a vegetarian diet. I know several dog owners who do this. You may have to do blood work, and be more observant about their health.
Note: Dogs, not cats, and vegetarian, not vegan.
I notice you were careful to make these distinctions in your first paragraph, but then (IMHO) went too far with "Your friends are literally killing their pets."
That is, there are several nutrients that cats have completely lost the ability to manufacture in their own tissues which are only found in animal sources (and whole animals, certain things are only found in certain tissues).
Vegan or vegetarian diets for cats have to be heavily supplemented with nutrients which are either animal-derived or entirely synthetic (and the type of people who want to feed their cats vegan food are also the type of people who have big problems with synthetic chemistry as food).
Dogs (and humans) do a bad job at manufacturing many nutrients with animal sources, where cats don't do it at all, we just do it poorly. Supplementation and careful diet design make it possible to thrive on plant-based food.
If you tried making your own plant-based cat food without a chemistry lab and a PhD, you'd be literally killing your cat.
If you tried to make your own plant-based dog food (or baby food) you'd probably end up with huge risks of malnourishment, but it's still possible with careful monitoring.
Cats are another matter. Unlike most other animals, they cannot biosynthesize taurine; they must find it in their diet, and it's mostly found in meat and meat products. As such, cats are obligate carnivores -- they cannot survive on a vegan or vegetarian diet.
Note: Dogs, not cats, and vegetarian, not vegan.
I notice you were careful to make these distinctions in your first paragraph, but then (IMHO) went too far with "Your friends are literally killing their pets."