HOW CAN A FRIEND RECOVER FORGOTTEN USERNAME?

PLS HELP!
A friend of mine signed up about 18 days ago. He didn't know he would wait up to 7 days before the verification can be completed.
So many days gone by and he finally stumbled on an email from steem with a link to finish the registration process.
My friend was able to get his password from that process but has totally forgotten username. we have been searching for any little possibility that could help open the account.
please if you have any idea don't hesitate to share, it will be very much appreciated.