Your daughter is 14. She's plenty old enough to manage her own social relationships. You must stay out of it entirely, not speaking of it to the friends, let alone the parents! If you get involved, you will humiliate your daughter and cause many more problems for her - and you won't help the situation at all.
Your only roles here are:
- to be a sounding board for your daughter.
- to discuss what happens when you snoop or violate someone's private accounts (today's version of reading diaries or listening in on the other phone
- to discuss that what people put on social media stays there FOREVER, and she can learn from this how NOT to put stuff out there.
Your daughter has a few choices:
- she can acknowledge that no good comes from snooping and using other people's passwords, and let this go. Sometimes people just vent privately and don't continue to feel the things they said/wrote in a moment of frustration or anger
- confess to the account holder or both girls that she saw the password, used it, and saw what they wrote
- take the high road ("When they go low, I go high") and continue the friendship, watching for signs of distance or cruelty from the others.
- break off the friendship, with or without telling them why.
No matter how this plays out, it needs to be a learning experience.
ETA: I just read your SWH, and it seems that you are determined to get involved despite all the advice not to do so. Here are my additional questions/comments:
- So what if the girls question her about why she stops hanging out with them? She can answer/explain, or she can decline, and they can deal with it.
- Yes, it may come out that your daughter snooped. So what? She has to deal with this.
- How do you know the other girls will tell their parents what your daughter did? Maybe they will, maybe they will decide to keep the adults out of it. Even if they make her out to "be the bad guy," so what? Isn't she kind of responsible for this? Actions have consequences.
- Why do you care so much about what the parents think of your daughter? Maybe they will think less of her, but she did snoop and use a password. Maybe those parents will stay out of it, maybe they'll think less of their own kids for being mean on social media and take steps to help their daughters understand the danger of putting stuff out there.
Is it possible, even probable, that you are far more worried that this is a reflection on you as a parent, like somehow you should have prevented your daughter from doing this? Why is it important to you to make parents think less of their own kids so that you can feel better about yours? Honestly, I still think you are making way too much of this, and you are going to just make a huge brouhaha for your child that she will never get out from under.