I just don't believe she is having a good time and wants to be there, wherever she is.
Perhaps he is manipulating her by saying he will turn her in too if she outs him for an inappropriate moment between the two. She doesn't want to get in trouble, so she goes with him for what she thinks is a day trip and instead finds that she is being abducted. Since the school was genius enough to give her a suspension because of the situation with TC, it would be easy for her to believe him when he says she would be in trouble for other things, too.
Or perhaps he convinced her that her family doesn't care about her and that he does, so she sees him occasionally because she believes him and can confide in him, but she is also hesitant because he is older and it can be seen as creepy.
He may also have placed a significant weight on her, like telling her if she leaves him he would commit suicide or something. She doesn't have the experience in relationships to be empowered to break away if he is manipulating her emotionally or psychologically. She wouldn't want to "kill him" by leaving him. So she tries to detach herself slowly, but not slowly enough. This is why she hides from him at her work and asks her friend to take her back home.
These are all just guesses... thinking out loud.
I cannot believe that a 15 year old girl would be so romanced by a 50 year old man (yuck) that she would run away with him thinking they will live happily ever after. She may have been swept off of her feet at the beginning and then realized it was kinda gross and she wanted to leave, but he won't let go. And now he has kidnapped her so she can't get away.