Yup,a LOT of variables in what kind of drivetrains two trucks have,even the same year,and depending on whether they are both 1/2 tons or 3/4 ,etc..
But if he changes everything,he could swap whatever tranny the donor truck has into his,providing everything is there he'll need ,like crossmembers,driveshafts,etc..but its a lot easier to put back what was there to begin with!..