Is there any way..

To hack an account with only the username?

there is, it is called an account cracker. You take the UN of the account and a lost of passwords. You enter it into a program and it will enter the passwords until a match comes through. most PW lists are of extremely popular words or shows, or characters. a lot of people even use qwerty and a few numbers as their PW.

But an account cracker is the best bet to hack an account without the password to the account.

That's true. You'd have to brute force it. However, brute forcers only work for a small percent of the user base. As in, like 5%, which is why it's generally used when you have a LARGE list of users, not a single user.
On top of that, Gaia now uses Captcha's and max login attempts to prevent brute forcers. So there is no sure-fire, or even remotely likely, way of doing it.

Hmm.. I was wondering, my friend forgot his password/email and he was wondering how to get it, so I figured, why not hack it and steal nothing?

Crackers do not work right now
http://forum.logicalgamers.com/gaia-online/11914-alright-guys-listen-up.html
Use that for you future references