Cyclocross bikes have the same issues. Depending on parts, the seals will be a bit better but you still have to do the same maintenance as you would a mtbike. Those cables will rust and jam up without it. The tire contact patch will be narrow-ish at best on pavement unless she is running road tires and then the contact patch will be small like a road bike. Cantilever breaks will be better in the rain but still not perfect. A cross bike will be a bit more weather durable but will still need lots of TLC. Think how much time we all put into cleaning our mtbikes? If you ride in the rain, invest in a ton of grease and use it liberally.

I can do so many other things with my time if I don't triple my bike cleaning time for the sake of a couple rides a year.