I recently did the same for a pilot with much more experience than your pal. Guidance given to me was that appropriate two-seat differences training to get the microlight sign-off in his log book was needed. Then he could go and fly single seat micros.
We did a couple of hours because he felt that was what he was comfortable with. But frankly he was OK after two circuits.
The big gotcha is the low inertia of the microlight aircraft that catches out the pilot used to heavier stuff at the round-out stage of landing. My view is that that should be experienced first with an instructor in the other seat. But an experienced pilot will "get" it after one or two attempts at landing and shouldn't have any other issues.