Is this a new install?
First thing I would ask is if the clutch is fully engaging......i.e...if it's a hydraulic clutch, then is the slave cylinder returning to its position efficiently....or is it binding on something? If it's a cable...is the cable adjusted correctly?
Also, tell him to check and see if the rear main seal on the engine is leaking....could cause some slippage if
oil is getting on the clutch disc.
Lastly, is this under full throttle operation or just putting around town? Could it be spinning the tires?