It would take you what 30 seconds or so to teach her the difference.
Thai dont use farang to describe any others but white people. Generally speaking. As has been pointed out they don't call chinese farang. When the word is supposed to mean foreigner and a Chinese person is a foreigner. Well it doesn't really hold up.
I'm not saying farang is being used with malice in all cases. However it is often unwillingly used as such. Taking into consideration that it's generally used for white people and not really to describe a foreigner.