Okay...
Determine the user's eight digit profile number by using the User Permissions cpanel.
FTP to your Members directory and download emailfile.cgi, then locate and download the .cgi file with the proper eight digit number from before.
Open both files in Notepad.
The emailfile.cgi file should contain a list of profile numbers, a character sequence (|!!|, |^|, or something similar to that), then email addresses.
The profile should contain about fifteen lines, the first being the username, the second being the password, and the third being the email address.
Check the emailfile address and the profile email address - do they match?