Neither, if he's keeping the plate he needs to fill out the form to put it on retention or transfer it, after he does that the dvla will issue a new v5, and that's the 1 you need to fill out in your name,
Filling out the current v5 with the private plate will put the plate in your name along with the car,
And filling out the v5 with the original plate will only cause probs as it's not the current v5, as the dvla has issued a newer 1(with the private plate) since that 1' so that's actually an old logbook and no use to anybody