Going back to our case of yesterday, if someone were to have continued ‘not going to the market on Friday’ after the market day changed to Monday, if his assumption is that his minhag applies in this way, then one shouldn’t inform him otherwise.
This is in keeping with what we learned in today’s daf, דברים המותרים ואחרים נוהגים בהם איסור אי אתה רשאי להתירן בפניו.
Since he considers that this is his “Minhag” and one must keep ones Minhag, if one were to inform him that it is permitted he may come to be lenient in things that he should not be lenient in.