Another approach might be to approach the vet (if you know who s/he is) for a friendly chat. They might be concerned to learn how their advice has been misinterpreted and may speak to your colleague again or supply pamphlets with accurate information about de-sexing.

I too get distressed when neighbours leave their cats locked out for long periods of time in all weathers. There is an old tradition of "putting the cat out" just before retiring to bed that many people still follow. Perhaps this practice comes from the days before kitty litter & cat doors were invented.

Most people will provide at least a rudimentary kennel for a dog but many expect a cat to find shelter & fend for itself. On really wet days I have brought my neighbour's miserable cat inside & towelled his soaking fur as he has nowhere to shelter when his people are out.

Conservation groups in Australia advertise in the media, emploring people to keep their cats inside at night because of the damage they do to our nocturnal fauna. People also need to understand that they must allow their cat access to adequate shelter from heat or cold at all times.

I feel like writing a short pamphlet about this and putting copies in people's letter boxes in the hope some will read it and change the way they treat their cats.