Well.... after spending about 30 minutes trying to input every permutation I could think of of their domain into the WHMCS license checker to get it to pass, and having it fail every time, in addition to multiple people also checking the same thing and getting the same result, their account was suspended for invalid WHMCS license.
So then they purchased a WHMCS license, and accused us of suspending their account for "no reason", citing their WHMCS license being valid.
They've been shown the door.