So, I did a little language research and the word Farang comes from the word "Frank" used to refer to "Europeans" vis-à-vis "Westerner"(stems from Persian and Greek).
Okay, so what! I am a Westerner, born and raised; therefore, I am a Farang.
I was in the Navy for a long time, called many things by my friends and those not my friends, it didn't stop me in anything I did in life. So, this guy should get over it and be thankful the Thai people have invited them to THEIR country and they haven't called him anything worse.
If he doesn't like it ... Go Home!
This Farang will not call anyone anything but friend and offer them a smile.