AFAIK, the fastest 486 was a 486/133 (33.3 MHz x 4) from AMD.

If it's a 600 MHz system, it's almost certainly a Pentium (family) chip so the 586 kernel will be fine.

If, on the other hand, it is a 486 system, then they should find a newer machine. Putting Linux on a 486 would be a challenge - they might have to find an old pre-built 486 distro, but I wouldn't recommend that route. 486s are very RAM constrained for modern OSes. Dumpster diving should get one a Pentium-class machine that would be better than a 486 for Linux.

HTH.

Cheers,
Scott.