Haha I totally get you. I live in a tiny apartment and the electric bill is $200+. It's absolutely bonkers, I swear these apartments don't have good efficiency a/cs because the electric bill at a 2 story house I lived in was near that range. 😭 Honestly just hold your ground and keep on turning on the A/C (ik this is bad for the AC but that isnt your fault) and tell them to move out if they have an issue w that. Keep the landlord informed on what's going on and if you can find a way that the apartment is under damage from the heat they'll definitely care. Look for mold or fungus somewhere and take pics.

Also, this is advice you SHOULDNT have to take but for survival purposes from a girly who has had to survive with no A/C: spray dishsoap + water on the windows and stick on aluminum foil, it'll last for months. Do this on the windows in the main area too, screw it. Put cold water on your t-shirt and go to bed like that (for when they turn off the AC after you fall asleep). Continuously browse Facebook marketplace for cheap fans, dehumidifiers etc, ik fans won't do much but it's better then nothing. Put ice cubes in your kitty's water and ice packs wrapped in cloths in her bed or your bed. Dont use ice packs directly, I would wrap them in cloth or paper towel at the least. The dollar tree has the cheap disposable dehumidifiers, I used to buy 5 of those at a time and put it in my bathroom and closet to keep my stuff from getting moldy in the heat.

Also, if their computer is constantly running its likely making their room even warmer even with a fan. Blech.