neighbour is never in to let her cat inside, what can I do?

hi guys, I'm just wondering if anyone else has had this experience, and if there's anything I can do for this poor cat beyond asking my neighbour for a key to her flat.

both I and this neighbour live on the first floor of a block or flats and both have cats, she lets her cat out while I do not since the logistics of letting a cat in and out of a building that can only be accessed via a fob doesn't make much sense to me. we're housing association, and we're not allowed to put anything permanent on the front doors of our flats, so she can't install a flap for her cat.

generally everyone else in the building lets her cat in for her and then we just knock on the door or ring her doorbell, but the problem is she's hardly ever in and the past couple of times I've let her cat in I've rang the doorbell and she hasn't even responded through her phone (she has a knock off ring).

I've just got back in from collecting a delivery and when I left my flat the poor thing was just stuck outside waiting. someone had clearly let him in and just knocked or rang her doorbell, she didn't respond as usual and then at that point whoever is letting him in has their hands tied. eventually the poor guy gives up and wants to go back outside. he probably wants to go home because he's cold or hungry, but he spends practically all day outside and can hardly ever get back in.

it's not fair on him, and I was going to ask my neighbour to give me a copy of her key so I can let him in, but at the same time that sounds nefarious in a way and I don't want her to think I'm gonna snoop or steal her stuff. I have my own stuff and my own cat lol. is there much else that I can do for this cat? it shouldn't be any of my business but she made it my business by expecting everyone in the building to make sure her cat gets in and out, without even asking first, and even then we can't get the cat further than the hallway.

oh and here's an edit before I can even post the thread: someone belled her from outside as I'm typing this and she's just gone to get a delivery so she was IN THE WHOLE TIME!! and she just ignored whoever rang her doorbell to let her cat in.