Oh yeah, when was the last time he rebuilt his diff.... that has a huge impact on performance. Also make sure it's set correctly, tight enough to be locked, but not binding. Thankfully the alloy tamiya diffs are a breeze to get feeling nice, whilst still tight. Have a look at this document... it's for the 416, but still applies to the 415.
http://www.thard.co.uk/TRF416-Things2know.doc
Oh, and check the droop, should be running 5/4 f/r outdoors (set on a gauge off the chassis bottom, measured off underneath the outer hinge pins).
HiH
Ed