Ah sounds like the trusty ignition lock failure. Try moving the steering wheel back and forth while trying to move the key. Tell your friend to order a new ignition lock tumbler matched to the VIN of his car along with an IGN. lock assy. I have had this part fail in all my MB cars.

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1983 Mercedes 300SD
1987 Mercedes 300SDL
2001 VW Passat 2.8 AWD
2007 OM642 Jeep WK 4x4