You can remove the clutch cover on the left side of the torque tube, then measure the distances between clutch sides to determine if shimming is required. It is an involved process since if it required, the tractor must be split at transmission and torque tube so you can remove the Power Director and shim the plates back to spec.
There is a simpler adjustment that may work for him depending on the state of wear. The cover plate has an adjustment to let the arm move fully forward till it's over center and if it does not get that firm feeling of going into place, it will return to neutral. That adjustment is clear when you look at the arm stop mounted on the diagonal studs protruding from the clutch over. First you adjust neutral with the bolts on the clutch cover adjustment arm loose and tighten just before you get creep. Then you adjust washer shims from the stop to allow it to move further forward (till it just snaps over-center, which apparently his is not snapping over center right now). If he can get by with that, it will save some serious work. The idea is that just the arm may not be adjusted correctly. It is worth a try.
The photo below is a D14 but the concept is identical. The bolt holes on the stop arm/neutral slot arm in the center of the picture, are slotted so it can be moved back and forth when loose. This shows the big stick (AC called the shift lever "the big stick" in their 50s marketing videos) in low, but you would put it in the middle detent position for locating the true neutral which will pull that assembly back and forth when you move the stick to locate the creep position and slightly back off before tightening. All of this and measurements of the clutch pack are covered in both AC and I&T manuals.