@Heliumeow-0 if the redirects only start once he submits his login details, the problem is most likely with his account, in this case he'll have more luck by sending a helpdesk ticket asking for assistance, in the main site.
One thing that I can advise you to NOT do is ask for his login credentials and trying to login from your IP. This would indeed be useful to confirm if it's a user-specific problem, but it would flag the two accounts in the tracker's system and the two of you would end up banned.
If you trust each other enough, you can try to test his login but make sure you use an IP you are sure you've never logged in from with your own account.