You say "she hasn't gotten used to it, and wants a Windows machine". Do you mean she hasn't gotten used to Mac OS X and wants to use Windows? Just use Boot Camp and install Windows on your MacBook, problem solved.
If you mean she is using Windows on your MacBook and there is something about the MacBook itself she doesn't like, perhaps you should elaborate on what specifically it is she doesn't like about the hardware as that's probably something you should take into account in your next purchase.
He doesn't know it, but underneath it all he means "She cannot abide change, and she is not competent to maintain her own environment in the stasis she craves.".
This is very serious for him. As he ages, she will become increasingly dissatisfied with life's changes, especially changes in himself. It will start with gentle digs about hair loss, and end with her throwing him out.
The best way to deal with this is to leave her now, before it all goes wrong.
Alternatively, gently persuade her buy and maintain her own laptop. The pain of holding even a single PC in changeless nirvana will force her to address her own psychological shortcomings. She will either learn to appreciate your ability to manage change, or she will have a complete mental breakdown and install Gentoo.