If there are any bylaws in your area about roaming animals (including cats) then I don't think if you accidentally fix someone's pet they can complain. They'd need to explain why their cat was roaming in the first place. Plus it would be free vet care for something they should have done long ago.
I wouldn't lose sleep over it, either.
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."