06 Chevy Equinox overheating when idle

Sister’s car back at it again…now I haven’t taken a look at the issue yet, that’s happening next week. But what she has told me is that it over heat’s at idle but not driving (I guess will be asking her that part in the morning) and she said the cooling fan does turn on (loud according to her). She also said it seemed to go down in temp when she turned the A/C on which has me confused as that’s the opposite of what is to normally happen. I need to do a coolant exchange on the vehicle in question, a oil change, brake fluid exchange and a few other things and still figure out the starting stalling starting just fine issue still since I have not had enough time to even remotely take a look at it due to my work schedule of overtime every week. Now I know the basics of the cooling system but I am posting here I think just to remind myself mostly of what it could be so I get the most accurate diagnosis possible. To me it could be a partially stuck closed thermostat since I have no idea when or if that has ever been replaced. Could be as simple as the radiator cap failing to hold pressure and she’s actually low on coolant or could be sever as a bad water pump needing to be replaced. Right now I just don’t know. I am hoping her boyfriend (not a mechanic but a long haul trucker) will take it into my shop tomorrow so I can get a quick look until I can swap cars with her and get her’s fixed. I am her only mechanic. Any suggestions you guys have is greatly appreciated.

Thanks in advance