Daspydyr 

I went back to pinned platforms and "Sticky Shoes" (5.10's Chase) on my mountain bike and would never clip in again in the dirt. I do have a roadie bike with Krank Bros egg beaters. I did take some tumbles getting used to them. I would encourage you to have your wife get used to riding a bike first and then introduce clipless pedals. A couple falls in the next couple weeks and you may never get her on a bike again. Take it slow on clipless.

I have seen several women with pinned platforms on group rides. Seems to be a good compromise. You do get better transfer of power, not that much. Plus your feet won't slip off the pedals, ever when wet. When you need to put a foot down, its instantaneous.

Plus, with platforms you can shift your foot around and change positions. Early on that can mean more comfortable as well.