Im sorry someone did that to your cat. But in the big scheme of things A. You were going to do it anyway correct? and B. Hey...you didnt have to pay for it!! Bonus!!!!
So in the end while I disagree with the fact that someone did it on their own without your permission I do understand why they did it. And seriously...if you were going to do it anyway and now it was free for you why are you so concerned about it. Go use that 100 bucks and have fun at the day spa!!