Can I assume you are getting her both pedals and shoes? I have recommended a more mountain/fitness/recreation shoe many times, but given this a road biking forum and I'll assume you are taking about road riding; so then I suggest a real road shoe and pedal. They are lighter, cooler (temp cooler), and more efficient for road riding.

So then I would not go with SPD. Its a nice system for the right application, but the tiny cleat on a hard soled shoe on the small pedal leads to rocking on the very small platform and hot spots are more common. Oh wait, my bad, I just reread your post. You use SPD so I would put her on SPD. This way you're dealing with one style cleat in the house. So if a bolt falls out you have spares in the toolbox for both of you. When she says somethings not shifting right, its easy for you to hop on her bike for a spin to see the problem. SPD pedals are so stupidly simple to clip into so into you won't find much else better. There are others that are just as easy in a different way, but SPD is really easy; well as you know. Plus there are so many great priced SPD style shoes and pedals out there. If she has never ridden clipless before, have her ride around the lawn the first time because she will fall over at the first stop. Thats just a normal right of passage.

Or......take the opportunity to get you both on real road shoes and pedals.

Speedplays are a very fine suggestion. My wife and I use LOOK and she's a novice rider but has no problems. I have new Sidi Veta's for her for Xmas.