Whosdaman: That's only IF his phone was rooted.
If not, a simple hard reset either by using the three-button method, or by going into Privacy and hard resetting that way. That will remove the Gmail account and any personal data. The only information that would still be in the phone at the point would be simple provisioning information, IE: the phone number, station ID that's attached to your phone number, and the data username and password that the phone uses to access Sprint 3G services.
To clear those, you would need to get the 6 digit Master Subsidy Lock to reset it. Once you get that lock code, you can use the dialer to enter ##786#, click Reset, then enter that six digit lock code.