I have several things that could maybe sway her opinion.
1- There's nothing like them.
2- Apple controls every little bit of the production of these things.
3- They currently do not have or get viruses!
4- They will last forever, my last ibook lasted 5 years and i sold it for $500.
5- They may be a little bit more expensive, but take this story into thought....
My father is a VP of a major (foreign) automobile company. They bought him a new dell about a month ago. He has had to replace the hard drive, re-install the operating system (three times), and get a new battery.... In a month!
I sent my iBook in once because it had an optical drive go bad in it. In five years, i sent it in once, after the waranty was up and they replaced it for free.
6- Popular Science magazine said that a macbook is the best PC out there (because macs can run pc)
7- my new macbook has Leopard, Ubuntu, Vista, and XP Pro installed on it. Runs windows better than my brand new (week old) HP i got at work.
8- Microsoft Office runs awesome and looks awesome on Macs too

9- Just take her into an Apple store, then take her into a Best Buy or something of the sort. Tell her to compare the service between the two. Put an Aluminum Macbook next to an HP at Best Buy. Compare the build quality of the two....
Really i could go on for hours. She'd be dumb to save a few hundred dollars now, when she'll end up spending the extra in repairs in the next year. Buy nice once.