(To start of, I'm assuming that the original poster understands that BF4 isn't available for OS X and therefore intends to use Bootcamp to play games on the Mac)
The Mac Pro is an expensive solution if it's meant just for gaming. Like others have been saying, you can get better gaming performance for less money by getting a Windows PC.
However, the statement of your friend that the Mac Pro will not be good for gaming because it has workstation graphics cards is wrong. See this article for example:
http://www.tomshardware.com/reviews/workstation-graphics-card-gaming,3425-15.html
Workstation cards (hardware and their drivers) are not optimized for gaming which means they are not cost efficient for gaming purposes. Again, you can get better performance for less money if you get a gaming card. But this doesn't mean that workstation cards can't game at all or won't still provide decent performance. The GPU's that will be included with the base Mac Pro (the D300) are expected to be custom versions of the W7000 (2GB instead of 4GB RAM). One of which, see the above article, should provide you with pretty decent gaming performance.
It's not really clear at this time if the Mac Pro will support crossfire under Bootcamp. If it does, even the base model can function as a pretty powerful gaming PC. But again, from a cost perspective, it doesn't make much sense to buy it
just for that.