First of all, I am SO sorry this is happening. How awful. FYI I am not a doctor.
I think therapy, for her and you, is an excellent idea. It might be a good idea for your husband, too. Maybe hearing from an impartial authority just what a bullet you all dodged will finally get it through to him.
If he won't go to therapy, then it might work to go to the local police station (observe Covid protocols, of course) to have them
tell him how things could have turned out.
Because even under the absolute best of circumstances (and you and I know they were far, far from the absolute best circumstances here), she would have been a child taken advantage of by an adult who for God's sake should have known better. And, truly, that should be enough for your husband to understand that this was a terrible situation and it could have gone very south, and it already was, let's face it.
I'm sure I don't have to tell you what the worst case scenario is, of course.
As for why your daughter did this, I trust a therapist will be able to give some insights and perhaps elicit an admission. But it could potentially have been what happens with a lot of victims, that they are told they're beautiful and mature -- things that either no one tells them or perhaps not enough people tell them. Or they don't believe it when others say it.
Being told by mom and dad that they look pretty may mean less (and they may even expect it in some fashion or at least feel like their parents are obligated to say it) than hearing it from a stranger who they have been led to believe didn't have to say it. And, therefore (to the victim's mind), it felt genuine.
And of course the predator is manipulative -- and a 13 year old child wouldn't even realize they were being used. No wonder your daughter is confused.
You're clearly a loving, attentive parent. I hope the rest of her teen years are considerably less eventful.