I personally believe the best policy is to state upfront that the family rule is: if mom and dad give you a phone or internet access or whatever, mom and dad have FULL access to how you use this technology. That way you're not "snooping" and it will cause them to think about what they write or do with the technology.
If the child balks at this, tell them you will take away the phone, computer, etc. It's their choice.
I mention this more for your younger children as they grow up and want all the same toys...
That being said, you've snooped and found some important information.
The problem is: you're the stepmom, she spends the majority of time with her mom, and her father already thinks you are too critical of his daughter.
Given all of that, I think you have two choices:
1. Ignore what you saw, stop snooping and follow the lead of your husband. Support him in however HE decides to raise her, no matter how much you may disagree. Of course, you can and should speak up about the rules you want for your 6 and 9 year old regarding current and future use of technology and dating and so on.
2. Take the advice from the pp below and just tell the girl, "how's your boyfriend?" I would also NOT say, "I looked at your text messages." You could say what the pp suggested or something like, "Oh, just a woman's intuition. I was 13 once, too, you know."
Try to keep the conversation friendly and open - see if she'll tell you about him - his age, where she knows him from, where she meets him and so on.
You could then simply tell your husband that you had a funny intuition that his daughter was seeing someone and that you found out you were right. I would then leave it at that - don't tell him what he should do and don't call the mom. If he decides to do nothing, then leave it be. Even if it means she finds herself in over her head.
I am not a stepmom, but I have friends who are and it is TOUGH. You sound like a good person, who truly wants to help this girl. But, as the stepmom, I believe the best thing you can do for this girl is to let the two parents work out how they want to co-parent.
My prayers are with you. M.