You don't mention the year or model of the car so i'll assume it's a relatively late model.
if she has a "smart" key then the reprogramming, as far as i know, needs to be done by the dealer. HOWEVER, if memory serves, you should be able to use the flat key to start the car. I believe that was the point of the flat key.
Your best bet, unless someone else in the forum knows how the system works, is to call the dealership parts or service department and ask them.
'94 W124.036 249/040 leder; 8.25x17 EvoIIs
'93 W124.036 199/040 leder; 8.25x17 EvoIIs, up in flames...LITERALLY!
'93 W124.036 481/040 leder; euro delivery; 8.25x17 EvoIIs
'88 R107.048 441/409 leder; Euro lights
'87 W201.034 199/040 leder; Euro lights; EvoII brakes; 8x16 EvoIs - soon: 500E rear brakes
'70 R113.044 050/526; factory alloys; Euro lights