It's irresponsible to let a cat outside by itself, doubly so if it's on a special diet. Blaming people for feeding a cat in such situations is like blaming a pedestrian for being hit by a car. I also mention this because outdoor cats are more likely to be hit by a car.

[–] cutemarshmallow@europe.pub 3 points 19 hours ago

That's true. I've been very tempted to feed it a few scraps of food (like breadcrumbs or leftover canned tuna), but I know some animals don't respond well to some food and I'm not used to cats yet. I had a dog so I know not to feed dogs any chocolate, grapes, or nuts, but all I know is that [most] cats can eat tuna so I didn't want to risk potentially making my neighbour's cat sick, or worse.

I have been researching animal sanctuaries to start the process of getting a cat though, so eventually I'll have some actual cat food. Maybe then I'll try to feed this one a few bites, but I think I'll ask the neighbour first because I don't know if this cat is allergic to a specific ingredient or anything.