Definitely make sure that his version reads ULTIMATE in the launcher. Even if he has an ultimate license key purchased, it could have not been entered correctly, and think it's a different version. Or it could be an ultimate key that where subscription has already been stopped (disabling is instant due to the way PayPal integration works).
That's the only reason I can think of that one person can connect, but the others not.
After that is checked, if still an issue, then have GM send a note to
[email protected] with a screenshot (Alt+PrtScrn) of the FG Settings window and FG launch window.
Regards,
JPG