Yes, definitely tell her father. She'll probably be mad at you, but 11 year olds do not need to be soliciting each other for sex on Facebook! There's only so much you can do as her dad's gf; her parents need to be involved. God forbid something awful happen and they had no idea what was going on.
"Maybe Lucy's right. Of all the Charlie Browns in the world, you're the Charlie Browniest."--Linus, A Charlie Brown Christmas
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