Does he have a bow mount trolling motor? If so just put it in the water and it will slow him down a few tenths. Turn it sideways will do a bit more. If thats still not enough turn it backwards and run in slow and increase speed to achieve the speed you want.
If he does not have a bow mount using 2 1×6 or 2×6 and c clamps can do the trick. Basically making poor mans trim tabs. Clamp each board to the transom so there is say 6″ below the bottom hull. fasten lower for a slower speed.
Just 2 thoughts to try prior to spending money.
There is always drift socks too but Im not a fan of trolling them in a smaller boat, especially tiller. They tend to take a lot of control away and want to steer for you in directions you didn’t want to go, at least in my findings.