thats horrible and inexcusible if you ask me - regardless if the vet "feels bad" about it or not. Its called check, double check and recheck - call and confirm with the client THAT MORNING or the night before what the procedure is. I know my vet does this.
I'd be wicked upset and would want amends if it were my cat. Like you, my cat is my therapy in a way. He's my little ball of furbaby and I've spent alot of money on him to keep him with me. He knows when I'm sad, he knows when I need attention or a nuzzle, or just someone to sit beside me and keep me company. I'll be heartbroken when he finally goes - but in the meantime...he's here, and if a vet messed up a surgical procedure on him.. OMG.. yep, I'd be extremely upset and want restitution.
Doesnt matter that the cat CAN live without its claws..its the point it shouldnt of been done, period. Sloppiness and incompetence is never an excuse.