The best way to do this is if your buddy is not more then 30 days past due is a esn swap to a dummy esn, PM me and I'll find you one. Past that, transferring a esn to a different carrier is a massive and utterly impossible if the esn is still attached to another account. If he can't pay off his bill give the phone back.

If you EVER buy a phone on eBay always make sure to get an esn and call Sprint to verify.