ugh, how can someone let it get so bad that their cat has sores? we took our poor late cat to multiple vets when he developed a skin condition, and i spent £600+ of my own money over the course of a few years for his initial checkups and monthly steroid when our current vet figured it out. the occasional flea treatment is much less in comparison, and while i get it can be somewhat expensive (we don't do it as often as we probably should, but we do do it asap for all of our cats as soon as just one of them starts scratching too much), it's better than the alternative of them developing a flea allergy or skin condition because of it.
is there anything your local cat charity or similar can do? i might be a bit wary of reporting the owner(s), since the risk of the cats getting sent to a kill shelter is arguably worse, but having someone potentially come to see their living conditions might spook them into taking better care of them. idk. it's fortunately not something i've had to contend with. if you can see the cats yourself regularly and keep an eye on them, that's definitely good though !! i love cats, so i really admire you for looking out for them even though they're not yours <3