I would call the parents to ask them if you guys can chat and ask if you can meet for coffee. Personally, I think this should be a face-to-face get together specifically so you can show them the text from their daughter to your son. I would focus mostly on your concern about her health and wellbeing and absolutely make certain they can see the text string between your son and their daughter so they really get a feel for where your concern is coming from.
I would also speak directly with the youth pastor at your church so he can keep an eye on her both from a sexual morality standpoint but also from a health and safety standpoint. It would also be a good opportunity for the church to begin a study for the young girls in the church on what modesty and purity mean.
"Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. This is the first and great commandment." (Matthew 22:37-38)