Me - I would pull the transmission and go through it. If the cluster shaft is sticking out 3/4", then it is my guess that the case has been taking a heck of a load in the rear area (and the shaft has been 'pushing down' with the front of it unsupported). The case shaft hole is most likely wallowed out and who knows what shape the thrust washers are in. Also, think about the potential for a damaged/cracked case, screwed up thrust surfaces, damaged gear teeth, etc.. In my mind, it is a bit risky to somehow knock the shaft back in (which I think will be difficult to get the lock-pin holes aligned) - as well as the shaft aligned with the front of the case. . . and then drive it any distance and feel 'safe' doing it.
He may very likely need a new case (and other internal parts) - or have to have the case bored out in the shaft area and 'sleeved' back to the correct shaft bore size.