If he needs someone to help fly his plane using his transmitter than he might consider converting to mode 2. But most modern transmitters can buddy box a mode 1 with a mode 2 transmitter so this should not be necessary except that...
His mode 1 transmitter will have the model memory and trim settings and this would usually be the master transmitter...
...and he would normally be holding the slave transmitter.
So someone would have to go to the effort of copying across his model setup...probably quite easy with SD cards
That said, he will probably not need buddy boxing because he will remember how to fly almost straight away