Oh I would definitely get something you can mount racks and fenders on then. I don't think you want a dedicated road bike.
Actually, in this case I'd advise slightly differently. She's already got a heavy ass mountain bike with fenders and racks. That'll be fine for commuting in shitty downpours and winter conditions . . . but now she wants a nicer light/fast bike to make the commute easy.
Get the road bike.
- Pare down everything you take with you to bare essentials. Keep your shoes, lock, spare flat repair kit/tools at the office. You need bare minimum clothes, and your lunch. That's it. I'm able to get a backpack down to about 7 or 8 lbs this way, which means that it's not too uncomfortable to ride with.
- You want to use a backpack because you'll be going faster on the road bike . . . which means you'll discover that rear panniers actually add a lot of drag. You are the other big source of aerodynamic drag on a bike, so make sure that you're wearing tight clothing that doesn't flap around.
- Bikes are lighter without fenders and a rack (and a kickstand). You will notice the difference in speed and ease of travel. If you
need to have fenders on during the spring/fall because it's wet all the time, then look into Crud Roadracer type fenders . . . they don't need braze ons and work well. Take them off in the summer though, you'll go faster without them.
This is basically the approach that I've taken with my bikes. I ride the winter/rain bike all winter and the start of spring / end of fall. It's fine. The rest of the year is pure joy though.