Originally Posted by
ari_one
Check all of the hoses and verify that none are leaking. Also a bad t-stat will cause it to overheat as well as a bad temp sensor.
if the t-stat was stuck closed or the fans weren't turning on, he wouldn't be eating coolant.
I am going to go with a leak somewhere or a headgasket starting to go. Both of these are easy-ish to diagnose. Like most said, run it full of coolant and look for where it is spraying out. You could also pressure test the cooling system if you have the tools available to you. For the headgasket you can do a leakdown test, again if you have the tools available to you.