I can only reply for myself... you'll want to let her pick out her own bike completely. Any bike is going to be uncomfortable as you get used to it, so she really needs to go through the buying process and decision on her own. It not only involves what her temperament is like (does she like exploring in forests, or going to pretty park, or is she the kind who walks around the neighborhood?) but also things like

1) how old she is (some of us over 40 find the newer bikes unattractive sometimes),
2) how much she weighs (apparently this can be a factor. Most of the Clydes here use a more modern, compact geometry--or a bent!)
3) What kind of bike you* have... (nothing worse than being with someone going twice as fast as you when you're on a 40 lb bike and working twice as hard)

I like old fashioned racing bikes myself. Most of the people here didn't start with one. But I do not like compact geometry. It just doesn't work with me. I don't feel like I'm "flying through the air" on a cross bike, but a lot of people here like it. Start with the handlebars at seat height, no matter what the sales guy says.

Let her choose, and tell her about us up here in Clydes/Athenas! There are certain things we've all gone through, so hurting wrists and achy knees don't have to sideline her.

If I were married and just starting to bike with my husband (and this is just me) I would want him to go at my pace, but not let me know he was holding back. I would want him to suggest taking breaks so I didn't have to all the time, and I would want to pick out my own bike, for sure. You don't sound like the kind of guy that would push her too much, but it's something you don't want to do unless she was a high-school athlete and used to it. At least, that's how I would feel if I was your wife.... but I'm not! You know her best...