[Fey]

The problem with confronting her is this(advice as someone who used to be a manager):
No matter how you present it, unless [we] see it with [our] own eyes it's your word against her's.
Yes, you may be right, but 90% of customers won't complain, so any negativity towards them is going unreported by the people that would cause the most impact. This is why the advice I gave was so specific, it'll let the problem stand on it's own. Unless you are her direct supervisor nothing you say will be nearly as affective.