If he has decided to go to a school then let him do so, wait until he has finished with them and then do what the syndicate have asked you to do. Do not try training him at the same time as someone else or he will be the one who has problems. The chances are that there will be differences in what you tell him compared to what both the other instructor tells him and the way he was taught. When you do come across these differences explain why you do it your way, hopefully the other instructor will have explained his reasoning and the candidate can then decide which way he wants to do it.
Many schools standardise the way they teach because it suits their aircraft, operation, and experience levels, it may not be the way a syndicate would operate.