the problem is that every light socket and plug/outlet is exposed to water. in each strand there are 102 potential places for current leakage. (100 bulbs, 1 plug, 1 outlet) im surprised christmas lights work at all on a GFCI outlet.
The only solution i can think of is to recommend to the customer that they keep the connections away from the ground and out of puddles. if theres still a problems, try splitting up the load among multiple GFCIs if it is possible.
my house is old and doesn't have GFCI on the outdoor outlets, so ive never had that problem even though i hang a couple thousand lights. unfortunately, i don't think it would be a good idea to remove your customers GFCIs