One of my neighbours makes me so fking mad.

She has an elderly cat who is at least 14 years old, and basically looks like a skeleton covered with furry skin.

He's been an indoor/outdoor cat ever since he was big enough to jump out of neighbour's apartment which is on the ground floor. But now he's too old and skinny to do that, so my neighbour lets him out via our building's main door. She once told me years ago, that he doesn't know how to use a litter box because he went outside to do what he had to. 

But she pretty much leaves him out all day every day. My bestie Rob (the real Murdock) told me that he was looking at the security cameras about a week ago as the police were out at another neighbour's apartment (immediately next to mine). He said this was at around 2 a.m. and the cat was sitting in the porch by the door.

Several times within the last month I have come across the cat curled up asleep in the long grass that grows along a bordering fence. Each time I paused to speak to him; one of the times there were flys on him which made me think he'd died. But when I said his name, he looked up much to my relief. Today we passed him on my way to and from the shop, curled up in the grass right beside the footpath. If someone with an off-leash dog were to pass him, he'd not stand a chance because there's basically nothing to him. Another time, Rob and I were on our way to the shop and the cat was curled up on the footpath by the front gates.

It wasn't so bad during summer because it was warm, but not that it's autumn and has been wet almost every day, I'm getting more concerned about him because he has very little muscle mass or fat to keep him warm.

I don't know whether to contact the U.S.P.C.A. or not. I don't know whether they'd do anything. He has a home, food and water (though don't know where he gets water from outside), and as far as they're concerned, that's enough. I'm not on speaking terms with my neighbour as she once said transphobic shit to me a few years ago, and I told her I didn't want to have anything to do with her.
I tried to find out whether or not it's considered neglect for him to be left outside all day at his age, and one person said that it'd be stressful for a cat that's been used to going outside, to be kept indoors.

Do any of you have an older outdoor cat or know of anyone who does?

you could build him an outdoor cat house since your neighbor won't take care of him and i doubt because he has a home, technically, that the authorities could do anything except take him and have her go to claim him and maybe pay a fine. there are tons of guides online; old tires apparently make good outdoor houses because they're sturdy and water resisitant. you just have to be careful what you line it with (blankets can freeze the the animal if wet, iirc) i have a family member with farm cats and she has little houses set up for hers with heating blankets in them for really cold nights. then paint it with the trans flag to really stick it to that old cu.....  lady xD so she has to live with that fact that her cat is dependant upon and taking shelter in a LGBTQ safe space.