Friend's Stampede 4X4 3s Set Screw

My friend just bought a Gens Ace Lipo 3s for his Stampede 4X4 and a fan. Came to my house today and not 5min, set screw came loose. Did this twice, then third time the pinion cut right through the spur gear. What can he do to have this set screw tight?? He's tightened it up real good but the 3s just is crazy in that small truck.

loc-tight it in place, just remember if you choose this route then 9 times out of 10 with a set screw it's there for good(depending on the compound of loc-tight)

WAIT! Make SURE you use the BLUE loc-tite. The red loctite is like super glue.

How about Monkey Glue?