You're facing that defensive thing that happens when people see others parent differently to them. People can discipline as they see fit (so long as it doesnt become abuse).

Ask her to drop it, there's nothing to discuss. If she keeps on, she's being disrespectful. You can walk away. If she keeps up, tell her you respect her right to an opinion. You disagree and would like her stop arguing about it.

It's the same as if you disagreed about anything else. Some people just wont shut up about it.