Tell him that at no time has Linux ever claimed to be Unix, and that this is a deliberate act. In fact, the technical name for Linux is "GNU/Linux" (though only Richard Stallman seems to really care). And "GNU" is a recursive acrynym which stands for "GNU's Not Unix".
After the fragmentation of Unix in the 70's and 80's, there are huge compatbility issues between the various flavors of unix-like operating systems. Claiming to be Unix is not necessarily a good thing.
Anyway, "Unix" is a trademark of the Open Group. You can only call your OS "Unix" if they say you can. And they have only certified Tru64 Unix, AIX, and Solaris. So BSD can no more lay claim to the name "Unix" than Linux can.
I've also heard the tired old arguments that BSD's memory managment is better and it's networking stack is more robust.
The memory management may be better. But if so, it's only marginally. If Linux hasn't caught up, it certainly will by the 2.6 kernel.
And although Linux's original network stack code came by way of BSD, there is no longer any difference between BSD's and Linux's network stacks. Last time I checked, for example, FreeBSD uses Linux source code for its networking subsystem. In short, for networking, Linux is the standard of the two.
And if he doesn't like RedHat, that's fine. He runs BSD, so he never has to worry about them.
In short, tell him, "This is my computer." And offer no further comment beyond that. ______________________________________________________________________
TANSTAAFL!