It is a sensitive area. You don't want to make her feel like she has done something wrong or that her body is shameful or that kissing Barbies are wrong.
I would make a comment to your friend saying that the other day it kjind of bothered you that her underwear was showing while she did splits on the ground when the dck father came to pick up. I would say it very lightly and just see what she says.
I think it is kind of antiquated to talk about being 'lady like'. If anything it is a better strategy to talk about how their body parts are private and wonderful and should be kept to themselves and that they have to take good care of them and make sure noone else sees them but their parents.

Otherwise I agree with zen that it is pretty normal and not much to worry about. You don't want to make a mountain out of a mole hill