Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If you trust them to dose, then just leave a little bit of ferts there for them to add to the tank.
If you are dosing dry you can leave the ferts in sealed containers.
If you are dosing wet and you trust this person to do it right you can leave written instructions: Dose 5ml of this and 8ml of that (or whatever you want them to do).
If you would rather that they did not touch the ferts, then turn the light down a bit (perhaps lift it a couple of inches above the tank) and turn down the CO2 a bit.