And the goal shouldn't be to force someone to change, but to convince them that changing is in their interest.
Maybe have her try your computer (set it up with something familiar like KDE) and see how she likes it. Have her try the web version of Office. If there's an app she uses a lot, have her try an alternative. If she really doesn't like it, install Windows, make it as clean as you can, and try to get her to change one app at a time.
At the end of the day, it's her computer and you're just setting it up for her. If she understands why you think something else may be better for her, maybe she'll give it a solid shot.