If I knew the cat belonged to someone else, I would talk to them about it in person - I would never just take their cats and get them fixed on my own. If they were just stray cats with no home, then I would have no problem bringing them to a no kill shelter where they can get fixed and put up for adoption. I would definitely complain to the neighbor about letting their non-fixed cats roam around freely like that, and perhaps ask them directly if they even plan to keep the cat(s) as a pet.
We confronted my grandparents neighbors who were letting their dog roam the neighborhood. She eventually got hit by a truck and they didn't want to take her into the vet, so they told us we could have her if we wanted. We had her for nearly 15 years after that, until she passed away from natural causes.
