So let me get this straight. She was afraid because he stood in her doorway once staring at her, then she started shutting her door. Then she could see the shadow of his feet under the door. Is that right? And she was 11?
I have to say that a lot of 11 y.o.'s have a big imagination, and are easily scared. It does not prove one way or the other that he had any designs on her. If she never said he actually came into her room, or made any other moves on her, then I don't think that's all that ominous, but if she had real concerns, she should have told her mom. Maybe she did, and Billie didn't take it seriously. So, I would like to know if it continued after that, like recently.
When I was about 12 or 13, my cousin, a young man about 19 or 20, opened the door to the bathroom while I was sitting on the toilet.. and did not say "sorry" or immediately close the door. He stood there sort of smiling for a full minute, it seems. (A minute can seem like an hour sometimes.) Of course he didn't see anything, but it scared the bejesus out of me at the time. I don't remember what my reaction was, I was probably too scared to say anything, but he did walk away finally. I ran out the back door and straight to my SIL's house which was right behind us and told her and her mom. They made me stay there until my parents came home. When I told my mother she brushed it off and said it was probably nothing and that I just let my imagination run away from me. But she made my dad put a lock on the bathroom door. In thinking back, my mother was trying to keep me from being frightened of my cousin, and at the same time she was taking it seriously. Smart woman, my mother. Still... I never completely trusted him after that, even after I was grown.