It's a common misconception that a "hotter" plug means a hotter spark.
What the term refers to is the plug's speed of heat dissipation. That is, a "cooler" plug will dissipate heat faster than a "hot" one.
It won't affect performance by burning the fuel more efficiently. Though a "hot" plug may not foul as quickly, your friend's bike is still running too rich, and as such is not running with top efficiency (and power).
Too rich is a fuel/air problem that requires a fuel system fix, not a spark problem.
