He should be able to view any synth using the Silverlight viewer, but he cannot use the Direct3D viewer (which is built using DirectX - a Windows only technology... also the reason that the viewer was ported to Silverlight which works on both Windows + Mac OS 10).
Neither is the synther available for Mac OS 10, as it is only a Windows application.
Since your friend has a modern Mac, he has several options. He can run Windows on his Mac in several ways. The most simple + straightforward is to use Apple's 'Boot Camp' application (which is built in to MacOSX) to install a second operating system... in our case, Windows. After opening Boot Camp, he simply selects how much hard drive space to give to the second OS, inserts an installation disc, + lets it copy the files + restart.
The second option is to use a Mac OSX application which acts as a virtual computer which will run a copy of Windows, allowing him to install the synther + Direct3d viewer.