You should probably elaborate a little more on your friend and his problem. Is he REALLY fouling the plugs, or does the bike run like crap, he swaps the plug, and then it runs great again? If the latter then he needs a new CDI, I have heard of several ’01 models with this problem (WR & YZ) and dealers in the know are able to get Yamaha to pony up for the swap.
If he really is fouling plugs then he needs to:
a) stop running it with the choke on
b) let it warm up before he rides it
Hope this helps!