Have her try flipping the key over. Also make sure it's the right key- does your 164 have 2 different keys for ignition/door and glovebox/trunk? Make sure she's using the right one. SOunds silly but you never know.

I've found some copy keys do not work so well on one side- improper cuts or maybe the key shifted a little bit while being cut, who knows. I've got one for my 244 right now that won't unlock the doors but works fine in the ignition. I've got another key that will unlock the door, but only if I first turn it in the lock direction. Weird stuff, and it's all about a few thousandths of an inch of material.

Have her try a shot of WD40 or something in the lock cylinder. And of course, turn the wheel both ways- apply some real force, to make sure it's not just the steering locking up the column. Sometimes in certain positions they can just be a real bear to turn. The 164 does have a bit of extra weight on the front wheels, and I can tell you it's NOT a fun car to drive without power steering. That was an ugly problem with my wife's first car (75 164E).

Good luck with it!
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Rob Bareiss, New London CT ::: Roterande Fläkt Och Drivremmar!