You already have a bunch of helpful answers so I just wanted to add on that when you speak with the friend's mother, you strongly recommend she consider purchasing Net Nanny (www.netnanny.com) or some other type of internet monitoring software. It's fairly inexpensive too ($30/year I think). It monitors, restricts and reports website use, chat rooms & social media, time, etc. There's parental controls so you can customize it.
If I were the other parent, I would truly appreciate the heads up. However, keep in mind that it's also possible that the other girls were either lying or exaggerating what happened to get a reaction out of your daughter. It's possible that they are upset that she didn't want to join in and they're trying to either make a secret joke to make her feel left out or make themselves seem cooler. At the very least, I would expect that this is the excuse they'll give when confronted, even if events did transpire as your daughter thinks. If the mother gets the internet monitoring software, she can confront her daughter with evidence and leave your daughter out of it (which would avoid turmoil with their friendship and your relationship).
I also wanted to add that it sounds like you should be giving yourself a pat on the back for raising your daughter to feel comfortable standing up for herself and not following the crowd, having a strong enough relationship with you that she can come to you like this, and that she's aware enough of internet safety (and obviously real life safety as well) to make the right decisions.
For any parents who are interested, there's two good websites that I use to teach internet safety to students in my computer classes: www.netsmartz.com (sponsored by The Center For Missing & Exploited Children) and www.mediaawareness.ca (it's Canadian). There are a variety of games, videos and activities for children of all ages. The Media Awareness site even has a section on advertising awareness.