did not think this would be such a squeaky topic..
I did watch the same 2 guys play again even more carefully, and this time 100% confirmed my previous observation, that the guy is doing it on purpose, and his friend just plays on without any problem
There is no squeaking when he goes to the net (split step) and volley. There is no squeaking during his service return and there is no squeaking when he takes small steps to hit ground strokes, and running all over for balls. It is only when he is moving back to the center after hitting a groundstroke that he does it, precisely at the time that his friend is getting ready to hit.
I agree that it is very difficult to prove, but I think a ref would warn the guy in a ref'ed match.