FIrstly thanks for helping your friend! I'm sure he appreciates you.

Unfortunately there is no linux version. It would be technically possible, and indeed, Orca does use some of our code. However, screen readers work at a lot lower level than many other programs - simplifying dramatically, but as an example, porting a word process between operating systems for instance, mostly requires making sure the system interfacing parts (like saving or printing) work and much of the internals can stay as they are. Most of what NVDA does is interfacing with the operating system to find out what is going on, where the cursor is, what has focus, what has changed, etc so it can present useful information to the user. In order to create a linux version, we'd actually have to rewrite the vast majority of the code. At this stage, we feel our efforts are best spent continuing to improve the experience on Windows, BUT should that ever change, we will be sure to publicise it widely. For now, Orca is the main screen reader I'm aware of on Linux, though I admit I have not looked lately.