I never allowed sleepoevers at that age. When we began sleepeovers it was only at my brother & SIL's house with a cousin or with one *very* close family whose kids were the same age. And when we did - the girls were at one house, the boys at the other. When they got older (5th grade?) I began to allow VERY limited sleepovers. My DD's BFF at that age had an older brother so sleepovers weren't allowed at her house since I had no contorl over who the brother's friends were. I trusted the parents and the brother but not any unknown friends.
As for this little girl and her desire to show her privates and see others'... this is not normal.. If it happened once it would be a nagging thing in the back of my mind - but something I could overlook. But - it's happened now a couple/few times and also at school. This child did not come up with this idea of her own. She's been exposed to something or someone inappropriate. Take your pick: uncle, cousin, neighbor, babysitter (or friend, uncle, cousin of babysitter....). Inappropriate stuff can be in person, on TV, DVD, internet, ipod, iphone, etc. Thereare sooo many ways bad stuff can get to our children in this "awesome age of information".
I would say something to the mom - casually since you're firends. Be prepared that she'll be definsive. She'll feel that you think her child is a deviant, or trouble, etc. And in her heart of hearts she'll wonder if her kid is too. But assure her that kids can see so much - suggest that maybe her child saw the babysitter's iphone, or from an older kid on the bus, a cousin, etc. Then back off.
One of your jobs as parent is to help your child make friends that are going to be positive for her. So while I wouldn't talk this kid down to your child, I would make a diligent effort to cultivate new friends. While it's nice to have friends who are upbeat, spunky and fun, it's possible to find ones who are also positive influences.
My teens have friends who they met in pre-school & kindergarten, and some who are newer friends. There have been many along the way who were left behind either becuase my kids shed them, or becuase I made a conscious effort to lose them. There's a 15 yr old on our street who reminds me of your daughter's friend, fun to be with, but had some disturbing behaviors. I made every effort to not be available for playdates when this child was around, and to ask my kid questions that would lead her to make her own conclusions about this kid years ago. I know, from this kid's mom, that she's constantly in trouble at school. And my son and his friends have told me the rumors that she's been sexually involved with many different boys at school. While I feel badly for this girl, I am also glad that my daughter hasn't been hanging out with her for many years. They say hi to eachother and go their separate ways. My son had a friend who also knew too many inappropriate things for a kid his age. Once was not a big deal, but after a pattern that disturbed me I just didn't allow him at our house, or my son at his house. The boy, and his mom frequently asked for playdates, and as they got older to "hang out". We were always "busy". Going to the movies with cousins, running errands, going to practice or a game, etc. I wasn't close enough to this mom to tell her of her son's disturbing behaviors (example - putting his hands on my daughter's and her friends chest when he was 8 and the girls were 12!!!) but I knew I didn't want my son to hang out with him! Eventually by middle school my son recognized him as trouble and I fully anticipate that he'll be one of "those kids" when he gets to high school next year.
I hate to be the bearer of bummer news, but this is a child who you want to grow away from. At this stage you have so much influence over your child and who they have as friends. As the years go by you have less & less influence, and their peers have more and more. You have these next 5-8 years to help them learn how to pick friends and by then they'll be doing their job of growing up and away from you. And who they hang out with by the time they're 15 - 18 can have such a huge impact on what happens in their life after that. Seems silly that at age 6 you're forming their adult years - but that's how it works. We only have a short time to instill our values before they temporarily lose their minds and become teens. Hurry!