If actually a kidnapping, get the parents' phone number from the boy's mouth.
Call to reassure them he's with us, and that me and my wife are deeply sorry for our little girl's behaviour, that it will never happen again, etc.
After the boy's anxious parents hurriedly come to pick him up, clearly unhappy about what happened, have a talk with daughteru about how kidnapping is bad, and if she wants to invite someone over she must ask said friend and us for permission.
If she does it again, she'll have to go to bed even earlier or some shit.