If his attempt at tightening doesn't help, he should make sure the tuner is actually being pulled back buy the string tension.
I had an odd situation recently. The librarian asked me to replace a broken string on one of their loaners. The tuner was turning with no tension at all, so I didn't think I'd be able to tighten the string. To my surprise, as I continued to turn the knob, the string tightened, and the tuner remained tight. I think the pull of the string on the tuner mechanism kept it under tension.