When her dad dropped her off, did she have any money? How much did they cost? Within a range she could theoretically afford? Is it possible she bought them?
You've said she denies stealing them, but what is the reason she gives for having them?
I guess my first step would be to make sure she actually did steal them.
Then, if that gets confirmed, I'd try to find out what is going on in her life to make her do risky behavior like that. Teens sometimes do things intending...