Here is what I'd do, If your cool with it.
Here is what I'd do, If your cool with it:
1. Boot up into HBOOT. (Turn off phone, take out battery, put back in, then hold volume + power until a white screen comes up.)
2. Scroll down with the volume buttons to FACTORY RESET Then press the power button.
3. The phone should reset and you should come back to a clean phone. THIS WILL WIPE ALL DATA ON THE PHONE
If you don't wish to do this, I have another method that will involve unlocking the boot loader (WIPES DATA)
then flashing TWRP and wiping the data portion of the phone.
---------- Post added at 10:26 PM ---------- Previous post was at 10:23 PM ----------
1. Go to Settings
2. Scroll down to accounts and sync.
3. Find the guy's gmail account.
4. Tap it and then tap remove.
It may give an error, but it is worth a try. Sorry for the overcomplicated old response... got a bit carried away.