Most universities are pretty open with faculties computer requests, so long as you make purchasing go though the IT department and you stay within the budget.

It might be the case that she'll only be able to get the money for whatever the windows laptop was going to cost, which given what group rates can do, is probably less than the cost of the mac. So she should be prepared to shell out a few hundred dollars at least. More if she's going for something big.

I think you say "it's what I'm most comfortable, and I have important work to do, so by god, do your job so I can do mine" to ornery IT folks, particularly if you have tenure.

If you need to cite applications that you're currently using. Easy things to mention: it's easier to run LaTeX under OS X/unix (she should insist that if they don't get OSX for her, that they should have to get freeBSD working and provide her with support...) heh. There are good qualitative data analysis programs, plus integration with R (an open source stats program) in textmate which might be helpful. If she can say "for x/y/z apps," that might be helpful.