The Windows Experience Index is a horrible benchmark and really shouldn't be used for anything more than a general idea of the computer's capabilities. For example, if one scored a 3 and another scored a 7, you'd know for sure which is faster. A 5.4 vs. a 5.9 is too close to draw any solid conclusions, really.
If you really want to compare, download the free version of HDTune and compare the minimum / maximum transfer rates (higher is better) and access time (lower is better) of both computers.
Chances are, your hard drive is a 5400 RPM unit and his is a 7200 RPM unit. His would be faster, but the difference won't be night-and-day.