Contrary to what some people think, you CAN marry a sear to an SP-89. You can tell by the indication on the Form 4 of caliber, barrel length, etc. After transfer of the sear, a simple letter faxed to ATF explaining the "divorce" is all you need. You don't need to SBR the SP-89 as long as you don't have any of the offending stocks on the gun when you remove the trigger pack and place it on another gun.
I would tell your friend to wait for the dealer to transfer the whole package, then when he has it, write the letter to ATF for the divorce. That would be easier than convincing the dealer otherwise.