It sounds like he installed the ROM without wiping the phone first – oh,
no-longer-favorite-nephew, that will never work.

The easiest way to recover would be to restore a backup in ClockworkMod
Recovery. However, if you don’t have a backup you won’t be able to do that.

Without a clockwork backup you need to boot into clockwork recovery and
select “wipe data/factory reset.” This will wipe your phone completely
and give you that “right out of the box” feeling. Seriously, you’ll have
to go through the configuration setup wizard and put all your settings in.

If you have all your apps backed up by some program, like Titanium
Backup, you can then restore them after you wipe.

Good luck!