How to respond to the teacher who puts the feet up while talking to student?

I've done my PhD in Japan, and have had much contact with professors in America and Japan.

The academic relationship and work culture in both countries are very different. In America it is generally not regarded as bad when someone puts their feet on the table while talking to you.

I had bosses and advisors in the US who did this. However, in Japan, the advisor-student relationship is more respectful and professional.

I would like to tell you that you should ask the professor to refrain from putting their feet on the table, but if they do it in their office, it is pretty much up to them. Since the office is the professor's space, I do not think it is your friend's business to ask them not to do it.

In the orient showing somebody your feet/shoes is a sign of disrespect.

A person who puts his feet on the desk in the western world shows that he (this is mostly done by males) is at ease/relaxed in an informal atmosphere. However, also in the west this behavior is normally only shown by people who are talking to their subordinates or peers at their own hierarchical level.

Your question is "How to respond to the situation". The answer is quite simple in my opinion: do not respond, unless you really cannot tolerate it in which case you don't really have a choice but to respond.