A heart to heart in my opinion will be best. If she feels close to you and is mostly an honest girl she should tell the truth. In some cares this type of thing can be a cry for help, and for some kids it's just the thing to do.

I'm a 16 year old girl, and have never sexted BUT I witnessed these things happening. A friend who was a guy, was a little to "over joyed" by sexting (as this was the first time he ever did it) and he told me EVERYTHING. All details came out even without me asking. I ignored him, shut him off, but he would keep talking and talking. I had seen glimpses (before I could turn away or close my eyes) of pictures his gf had sent him, I heard stories, conversations they had on the phone in explicit detail.. and I worried that if he said these things to me (a trusted individual who wouldn't go around spreading it and making rumours) who else would he be saying this too. I told this girl about it, and as far as I'm aware she stopped sending him pictures (I know because he went off on me about this), but I don't know if it stopped her from doing anything ever again.

Sending pictures is such a dangerous thing.. Teens should really be more aware.