Just login to FTP using the username and password you gave him. Just setup an entry in your FTP client for his subdomain and you're in business. If you forgot it, goto "Users" then "Passwords" and have it e-mail to you.
It sounds like what you want is to have a directory structure like this:
Where bob.deadtime.net is both Bob's FTP root and HTTP root. It's not a good idea to try that - the home directory of a user will be his FTP root - and his home directory also stores his e-mail and the log files for any domains in his directory.
Now you can have this:
You have to create the user first, and make his home directory "/home/your_username/his_username". Then add the sub-domain, and for the web directory select "/home/your_username" from the drop-down box and then type in "his_username/bob.deadtime.net".
Then you will have to deal with file permissions and thats why it would be less hassle to just login using his username and password.