A high resistance in a wire won't cause a GFCI to trip. Yes, a wire could be crimped and damaged but ANYTHING that causes leakage to ground should trip a GFCI. That could be bad or old insulation on a wire or dampness that has gotten into a crack in old insulation and other things. When your friend measured the resistance to ground what was the reading? Also got a question....120 volt lamp? If it's a low voltage lamp then there is a transformer in the system. Yes, next step is replace the GFCI.
Al