Hmm that's odd. I was about to advise to use your email address to recover your password and account but the site asks for the username and email address. The absence of the username will not allow you to recover your password. So in your case where you forgot your username, that's going to be hard. maybe you need to send a ticket for that to help you out.
If he doesn't value his old account's username or levels he could just make a new account and it's not against the terms to have a second account.
@bhaby Try to look for old messages if your account's email address was verified that helped me recover my username.
Firstly, the user did not said he forgot his username cause his message was not specific, he only said he forgot his account which could be only his password or both.
Secondly, the last time I check opening of two account is against the site rules and regulations."14.1. Having more than 2 accounts is strictly forbidden(by person/IP/house/family)." The best option is for him to check his old email as you said.