I have to disagree with this teacher, the biggest difference is that "Ni Hao" is Mandarin and not Thai. And unlike "farang", no matter how many years later or origin, it will still be Mandarin and not Thai.
I'm Chinese-American and there have been plenty of time when random people in the US would make random sounds to me, making fun of me, knowing full well the random sounds is not Mandarin. Yes, context and who says it also makes a lot of difference. For example, if the person is a good friend, it's can be a good tease and joke; but not when it's a person I don't know. (If you're curious, I did think it was funny when The Rock did it many years back.)
Anyways, I think for a Thai to hear someone say "ni hao" is the same thing.
I know most "farang" tourist can't tell Asians apart, also true visa versa, but still, speaking Mandarin just because someone is Asian can definitely feel offensive if one is not Chinese. I'm sure the Russian teacher have felt offended when people assumed he is American just because he is a white.