when you give the mussar you shouldn’t give it over as mussar. you should say it with gentleness and love and be sure it comes over from the heart not in a tone of rebuke. of course we’re obligated to give mussar even to strangers but this is how it should be done. no one will bite your head off for correcting them. (they don’t want to get arrested for assaulting someone giving mussar.) hopefully they’ll mend their ways but once you corrected her you’ve done your part.