I believe your friend has 2 options.
If they have an intel mac, they can create a partition for windows, and dual boot into a windows installation. I do this on my MacBookPro laptop and it works well.
or
They can run Parallels (or another VM app), install windows into that, and play that way. I do this on the mac computer at the office and it works very well. Also have a test shard set up the same way on a Mac Mini and it works equally well.
GhostRiderGrey