To see the member's username, go into Member Inquiry or Phone Operator and click the Online Banking button.  Then click the Display Username button.  Just like if a member forgot their account number, you'll want to follow your normal procedures for verifying the person's identity before disclosing the username.

Or if you wish, you can clear the username entirely by choosing the option to change the member's password Tool #14 "Member Personal Banker."  Then use the "Display Username" button.  The member would then need to log in using his or her account number and set up a new username online.