Should I confront the parent pot-stirrer?
She has a right to complain to you, but no right to stir the pot w/ other parents. I've only recently begun to see the need for a rule to cover this - parents who criticize coaches and even players with other parents. It can be toxic, as I'm sure you have seen over the years. To answer your question, I would talk with the parent and reassure her that you will hear with an open mind any complaint that she has, but that venting to other parents will hurt the team and her daughter.