They can try the following options:
Checking their proxies.. I believe you need to have them off..
Making sure that the firewall isn't preventing the page to load..
Try logging into another browser and seeing if that works.
I'm sure there's more but that's what I get from the top of my head. They can also try logging into the game from a mobile device and seeing if that does anything.
Keep in mind that you would have to log in to the page twice because it refreshes after the first time.