I was thinking about this last night. It might be something like the mom found him on MySPace, met up with him (heck, she might not have even said who she was until she saw him in person) and he was pissed off about it and lied to the police to get back at her.
BTW: That's a Michigan driver's license on a chain around her neck. (It might be just a state ID card, if she doesn't have a license.) I mention this because I've known people that, when the police suspend your license, they punch a hole in the corner of the card. That way, you still have ID, but police can tell you're not supposed to be driving without having to look you up in the system.
Of course, it might be an old ID that she punched herself...but I have never seen anyone wear their license on a necklace before...