Did the PC people upgrade to WinX (joking) 10..
wireless drivers for 10 are a pain.
Get them to delete all their current drivers and download win10 drivers direct from the manufacturer of the wireless card..
And they should still cross their fingers that it will work.
Frankly the best solution is to buy a cheapie TP-Link etc WAP for PC.
There are enough differences that PC will generally have issues with apple routers since 8.1 and the new 10.
Intel wireless cards seem particularly prone to issues.. look at the wireless card in most laptops. Notice apple uses Intel processors, Intel bridge chips.. but not a single intel wireless card to be seen. wireless is still at the voodoo stage.. and so getting things working across brands is still loads of fun.
If you want to try..
and replacement drivers don't help.
1. Can the PC join the main network?
Even if guest fails is there movement on the station if they get full access??
2. Is the wireless naming short, no spaces and no illegal characters. Apple names are problematic.
3. Passwords as above. 8-20 characters pure alphanumeric..
4. Do a test though with no password.. and control the channel.. depending on where you are in the world PC have channel issues.. test on 2.4ghz with 11, 8, 6 in that order.
If this test fails.. no other changes will work.. it is sufficiently incompatible that life will never improve.. welcome to voodoo land.