Move quickly. I had a personal plate on my motorbike. I applied to move the plate from the bike to a recently acquired car, and found that the DVLA would not allow the transfer as the bike had been on SORN for more than one year. So I had to tax, insure and MOT the bike JUST to get the plate transferred. This was last month.
If your friend's vehicle has been on SORN for less than one year, a transfer or retention is fine. Greater than one year and he will have to tax, MOT & insure it simply to transfer the plate. Ridiculous state of affairs.