He's a manipulator, pure and simple. And unfortunately he has a type, young minor girls. They are more easily manipulated as they haven't had enough worldly experiences to allow them to see relationship red flags. I am sure she feels they are in love. I am sure he has told her how beautiful and mature she is and that nobody in the world could possibly love and adore her as he does. When she returned home, she likely didn't know she was pregnant.
When she discovered she was, she probably told him, and he was probably thrilled. Nothing further ties a minor girl who thinks she is in love with him like a child does. Now he begins feeding her the I can't wait to be a family with you BS.
I don't think he will hurt her or the baby. But I think they will be more difficult to catch this time around. Last time was a lesson, and I think they've learned it. They will stay better hidden. She probably thinks he is risking all (breaking probation) and going on the lamb with her illustrates his undying love, look how far I will go for you and our baby.
Barf. Eventually the new and shiny wears off and she discovers he has her and the baby completely isolated from everyone, and then the abuse will begin.
I won't go into her parents not knowing she was states away for months and thinking she was spending the summer with a local friend. I don't know enough of the backstory to judge that. I will say it gives me pause and causes me to wonder what home looked like.