It is possible to get these things on a Mac. I know a lot of people seem to be under the mistaken impression that Macs are somehow immune to all viruses and whatnot, and they usually end up getting a bit of a rude awakening like this.

Anyway, just google DNS redirect and you should find some info on dealing with this pest. And tell your friend that using a Mac is not a blank check to do stupid things on the Internet. Just because you live in a nice neighborhood, doesn't mean your place will never be broken into. They still need to be careful about sites they visit, things they download, emails they open, the works.