Turned out my problem was more esoteric.

Does his key still have the "spring loaded" feel when he goes to the Start position?

If it does not, the problem is probably the mechanical interface between the switch housing and the electrical part of the ignition switch.

To assist in your troubleshooting, remove the key lock and attempt to turn the switch with a screwdriver. If it works, your problem is in the key lock. If it does not, it requires the following.

To fix, it requires removal of the complete switch housing. A PITA to be sure but if I did it so can you. See many experiences using the "Search" function (including mine). Once out, three screws hold the electrical section to the back of the housing. Inside, the mechanical section has two small "keys" that engage two keyways on the electrical section. In my case,those keys were broken off thus enabling the switch mechanism to turn without moving the electrical switch. The failure was probably caused by repeated and heavy-handed attempts to turn the key as Dear Old Dad has done.

If you need to replace the housing assembly, $200 from the MB dealer not counting the electrical section and the vacuum switch portion. It is shoppable on-line. I paid $145.00 complete with everything but the key lock from **********. Happy Hunting and please keep us posted so as to fill the data base.