Hello Thomas,
[Topic moved here.]
It can't be a PowerMac if he's using Hauptwerk v4, since Apple discontinued them in 2006 and they only work with very old versions of OS X (10.5 and earlier, whereas 10.13 is the current version), and Hauptwerk v4 isn't compatible with them (or with OS X 10.5).
Perhaps it's actually a Mac Pro (which was the PowerMac's successor)? Sometimes people confuse the two. Assuming so:
The first thing to do is to make sure that he has all OS X updates installed, as well as the latest versions of the drivers for all hardware (especially his audio and MIDI interfaces), checking on their manufacturers' websites, and that he's using Hauptwerk v4.2.1.
Once that's done, and if it doesn't solve it, and if you'd like us to have a quick look at a diagnostic file to see whether it gives any clues (although I think it's rather unlikely), please feel free to send us a (fully-current) diagnostic file from him after the issue has occurred, ideally with the St. Anne's sample set loaded (since we all have ready access to that).