In short, no. The system is too new for people to have developed any hacks or bios flashing tricks to get PC cards running in OS X.
You could very likely run it in Windows, but it'd be a crap shoot if the system would boot to OS X with it installed. Some cards seem to cause kernel panics in OS X, some seem to be ignored and treated like plain ole' VGA adaptors. Not sure how it would react to the PC version of the 7300.
I'd say you'd be better off letting him return it and getting a Mac card for your Mac Pro unless you are getting a really really good deal.
But, I don't have a Mac Pro (hardly anyone does at this point) so I could be completely wrong; it's happened before. Once. 😉