Depending on the course, the difference can be anything from minor to all the difference in the world. In general, yes, there is a difference and the road is easier. How big of a difference depends on many factors. How long is the grass? How uneven is the surface? How difficult are the hills? Just a few questions that instantly come to mind.
If possible, try taking her to places where she can get at least some training time in on the grass. That might help her adapt to the surface and build up the strength in the feet and lower legs to handle the softer and more uneven terrain.