I see Win 7 Home Premium on Amazon for $92 shipped. Crossover is a little cheaper, but like you said you know that Boot Camp 7 would be fully compatible.
It's a lot more work to install Boot Camp initially, and as long as he doesn't use it for anything other than reading the PDFs he won't need AntiVirus software, but you can always get AVG Free if he plans to go to other web sites etc while booted into Windows.
The booting back and forth can be kind of a pain though if he wants to do more than just read the PDFs and surf the web, YMMV.
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