Sounds like your visitor might be pregnant - if she is not spayed this is very likely. Some vets will do a pregnant spay depending how advanced the pregnancy is, might sound horrible but apparently sometimes this is necessary because the mum is too small to safely carry and deliver a litter.
Also some Cats Protection branches offer vouchers to help towards neutering, not sure if the PDSA also help with this but sure someone who is involved in rescue will confirm. If she is pregnant and needs veterinary help with the delivery this can be very expensive.
As she has a collar is it possible her owners do care, but just don't realise what the responsibility of a young cat entails? I used to feed a neighbour's young cat which was pregnant and ravenous. They were the most lovely family so I honestly believe they were unaware that she needed so much extra food, and the risks and potential costs involved in letting a youngster breed.
Perhaps your neighbour would appreciate your concern and support? If you move in five weeks you don't have to worry so much about the owners getting the hump!