Did you mean that he already split the case and removed the pressure plate and clutch? When tractors (and standard tranny trucks) sit around for awhile, the clutch spline can rust a little around the transmission input spline--especially down here in the hot/humid south. The method I use to free up a clutch: Start the tractor in neutral, (get the tractor warmed up). Shut it down and then start it up in 1st gear with the clutch pedal pushed down all the way. (obviously not in your barn or with something like your lawnmower in front of the tractor) Keep the clutch pedal in and quickly throttle the engine up and down until the clutch spline gets unstuck.
If the fellow already split the case then he can thoroughly clean the splines on both the clutch plate and transmission input shaft. I use sparing amounts of anti-sieze compound on the splines--but not too much that it will get on the clutch/flywheel/PP surfaces.
Let us know what you find out.