Hi OP. You will find that for a year or so or more between the time your child is 0-16 you need to become very Internet savvy and also keep a check on her Internet history. Tell her you will be very suspicious if it seems to get deleted on a regular basis and that you will delete it once you've checked it. Make it a rule of being allowed Internet access.
There are a hundred million pitfalls for young teens on the net and they're bound to fall into a couple as they learn to negotiate the Internet and find places where they are comfortable and can make friends. You won't be able to foresee and forewarn them (that's even if they listen to your warnings) but if you watch and check regularly you will be able to head trouble off at the pass or mop up messes before they go too far, as you are doing at the moment. Well done.
It's hard to do right for doing wrong and I can understand your reservations concerning getting your daughter's back up by going to her friends' parents about the present matter. It's a tightrope walk for parents as to whether doing taking something like this further will prevent her from coming to you in the future with when she's concerned about something that's happened in case you 'make a fool of her' in front of others. Our initial concern is always to think of our own children. However, as everyone else has said it is very important that you alert other parents. You could do so without giving too many details. Perhaps warn them that 'whateverhisnameis' may have approached their children and that he is up to no good. Cast your daughter in a good light in their eyes by saying she came straight to you (even if you are bending the truth a little).
Ask any young teen and most of them will have already spent an evening, with their friends or even without, on Omegle, ChatRoulette or a hundred other sites like those where they can see stuff that you would never dream of in real time with and without sound. A favourite is holding up little notes (you could keep an eye out for these in waste paper bins or crumpled up under the bed). At that age most of us would have been up for the giggle of seeing a variety of penises or boobs in full and glorious technicolour which is pretty much what most of those sites seem to be about. For most teens it's pretty harmless entertainment - there will be those though for whom it's a lure. The more you can find out about the various modes of social media the better and if you feel you can speak openly about them to her all the better.
The world has changed for our teens and if you don't change and keep up with them, you will have no hope of knowing or finding out what they are getting up to. If you don't know much about the Internet and I'm not talking about booking travel, find yourself an older teen who will spend an evening with you navigating sites like that - just don't get yourself arrested and make sure the older teen's parents know what you trying to accomplish with them.
Before Christmas I had to run off a groomer who had contacted my 12 year old boy through a Youtube video he has posted of himself driving and they had gotten into long conversations about cars - within 48 hours my son had given this chap (supposedly a 31 year old school caretaker from Portland, Oregon) his mobile number, and yes, I'd spoken to him endlessly about stuff like that but sure in one ear out the other as soon as somebody adult, 'nice' and willing to spend ages talking to him (unlike most adults) came on the scene. I had to explain to him that no adult who was on the level would be interested in speaking to him about cars like that for hours at a time and that if the adult was on the level he certainly wouldn't be asking him to take photos of his feet on the pedals (a foot fetish or working his way up, who know!). He was really gutted that he hadn't made a new friend but when I asked him did he know any 30 year olds in real life who would be interested in talking to him for hours about cars he had to admit that he didn't and would find it strange if they did. This guy, coincidentally, had a friend who worked in Nike and could get runners for free sometimes and what was our address? Bastar
d! I also had to explain that this chap could be living up the road, up the country, in the UK etc and might never have been near the USA in spite of the photos on his FB page. I had to explain paedophile rings etc to him.
It can be very difficult to say no Internet to them so the next best thing is to be one step ahead of them or alongside them and with full access to their electronic media.
Sorry for going on...