This was an earlier version of minecraft so the code base has changed a bit. Setting priority would avoid contention among processes, but some applications don't play extremely nice with this. He definitely has an earlier Intel, but it's never stated which one. If he has a Pentium D or P4 the second processor would be an HT core. Early versions of HT was awful and it's only useful when threads are blocked. (Like disk access). If memory serves HT support changed a lot from XP to Vista/Win7. It was pretty awful back in the day and typically I disabled it in many systems.

So, yes, depending the makeup of his system there could be a big boost in removing the second processor from the application.

In most systems today (coreduo, core2duo, core2quad, i3/i5/i7, sandy bridge, ivy bridge), this will make minecraft run worse. Changing the processor affinity from all to 1 core is reducing the amount of resources that minecraft can use. RowdyPope is also correct in the points that he mentions.