I would like to be more specific in my response to you but I couldn't log into ALLDATA for some reason. Any how the aux. fans are pressure and temp. actuated. Pressure from the A/C system will engage a low speed setting on the aux. fans and I think (not sure of the exact temp.) about 107'C engine temp. will activate the high speed setting on the aux fans. Other car brands are switch activated as soon as you turn on the A/C system but not the Benzs.
The air in the system could very well be keeping your friends aux. fans from coming on. An easy way to tell is to ground the pressure sensor and see if they come on, the sensor is behind the left headlight on the A/C line. Can't miss it!!