<A HREF="http://freshmeat.net/projects/memtest86/?topic_id=861,146" target="_new">memtest86</A>
<A HREF="http://freshmeat.net/projects/cpuburn/?topic_id=146" target="_new">cpuburn</A>
<A HREF="http://freshmeat.net/projects/diff_memtest/?topic_id=136" target="_new">disk and memtest (different to the above)</A>
Those are all really hardware testing apps, but if he's running a minimal "standard" (+factory patches) RedHat, it should be pretty solid itself. I'd expect some hardware to fail sometime, so hopefully that's been considered too. Eg, hotswap PSU, HDD, for example, along with appropriate software support. You could dig out some network testing apps too, and see how well it handles large traffic volumes on the NIC(s).
<i>Knock Knock, Neo</i>