Cats require taurine, which cannot be synthesized and is found in meat. Humans don’t have that requirement. Sounds like owner is an ignorant idealogue.
Rush brought up this very subject during the early years of his show in response to a story about some nutbar turning his dog into a vegan. He pointed out that dogs mostly only eat vegetables when they’re sick, to make themselves throw up to feel better and expel something bad they ate. He then went on to mention that if some moron tried that diet on a cat, it would die. He mentioned that cats live off of a protein that’s only found in meat.
Vegans need a B-12 supplement which they cannot obtain through strictly non-meat edibles.