Security through Obscurity is not Security!
What it might do is reduce the noise in your log, since you won't see all the bot traffic probing on 22 and trying to if your ssh open with user/password, etc.
If this guy wants to be secure - I would move him to vpn to be able to ssh in with MFA that makes he jump through like 15 hoops and has 5 seconds to enter his code and then has to ssh from the box you let him into through 2 other boxes inside to get to the box he wants to get to ;)
Then he will feel secure ;) And make sure his passwords change every 3 days.. And he has to get a new cert for his vpn connection every other day..