Magnet Mom raises some good questions and points about what can happen when trying to communicate with other parents about their kids.
But...but.

Devil's advocate maybe, but: what if this girl doesn't know what she's doing? Which is quite likely, considering her age. She's definitely putting herself at risk with this behavior and it could end poorly next time. What if you were the tipping point to nip this in the bud? Just something to consider.

From what my small-town parents tell me, back in the proverbial day, kids couldn't get away with half as much when almost any adult might inform on them to their parents. I have very mixed feelings about this apparent shift towards people 'minding their own business' all the time.

Most of all, good luck communicating with YOUR child.

What did the MOM, say???? Did she know, or was the girl being "sneaky"???? I am REALLY concern for MY SON, these days????

i would tell, i just told a Friend about her 12 yr old walking down the street dressed like a 21 yr old. she didn't get mad . you have to stand up for the kids ,to keep them safe not saying anything can get another child hurt. girls are very aggressive and dress way to sexy and say sexy things. i will always say something to keep kids safe . maybe because i have been a apartment mgr for 16 yrs and i have seen lots so stuff that made me mad and upset . yes yes yes you should always tell

OKAY, i know i am 5 days late, What did your son have to say???? I know from my son,...GIRLS are very aggressive today!!! He had one break-up with him, because he would not have sex with her.....When i saw the "girl" and her family, at a "TOWN FUNCTION" The Apple FESTIVE-ABLE", They did not seem to care their teen daughter was "half-dressed"????

yes i would tell her parent about it .my daughter was getting text messages from a boy about sex i took her phone and contacted his parents about it so yes i would tell her parents that would be wise.

WELL WE ALL DO THINGS DIFFERENTLY SO WHATEVER WORKS FOR YOU. I WOULD TELL EVEN IT MEANT LOSING A FRIEND MY CHILD IS MORE IMPORTANT

I think MagnetMom said it very well. I couldn't have said it better, so I'm not going to try. :-) That's great advice.

don't punish your son for her actions.i would take the phone and show her mom. young Lady's are very sexually aggressive theses days . sure don't need a baby this young

Notifying the girl's parents is always a risky proposition. You might find they don't care, or aren't receptive to your letting them know. Worse, you can't predict how they'll react to you showing them what their daughter is capable of. They might try to blame your son.

That said, as a parent, I'd want to know if my child was acting inappropriately.

If it were me, I'd discipline my child and the cell phone would go--for ages, and I probably would not tell the other parent unless they were a close friend (and I'd have to be willing to lose that friend as well).

I know that's not a perfect answer, but disciplining other people's kids can be a real touchy subject, and I know I'm not up for the headache.