Right thats true it could be your clutch isnt disengageing fully. If that was the case wouldnt he have trouble upshifting as well as down shifting?
You can try shifting when the car is off, if you can shift smoothly then I would probably look at the clutch first, tranny next. Fortuantly when the clutch gets looked at the tranny is dropped =) Of course its $$ to do either one though.