Try these threads:
Blower motor replacement procedure for 300E
Nothing blows from center vent
winshield fogging up!!!
It includes a description of which position the buttons should be in to make things work properly.
MB AC systems are not very intuitive. On some models apparently cold air doesn't come out the side vents, depending on the position of the pushbuttons. And to compound things, on some models the AC controls are different in the US than in Europe or Asia. Good Luck on the AC. If they don't have an owners manual with the car, maybe they can find one somewhere, as there are a number of other little wrinkles that are cleared up by the manual.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E