What even is sexting?
Sexting is sending photos of you or someone you know naked or near it. It counts as pornagraphy, and is illegal, for youth, at least.
How could you support someone? Well, listen to their worries, and a lot would come from sexting, and let them explain why they did what they did. Being supportive by just sticking by their side, or, encourage them to do what's right, such as if they haven't actually done anything yet, convince them that they're better off not to. As well, you could share what you know about the subject.