We don't actually know for sure if one of the answers is true. If one of them is true, it will probably the first one, because if it was actually stolen, she wouldn't have any reason to lie to her father about a friend bringing it next weekend.
What the two different answers tell us (and Mr. Brooks) is that she lied to the police, when they question her about the murder at her college. We can also assume that the car was somehow involved in the crime, otherwise the police would not ask for it.
We don't know for sure if she really committed the murder, but it is strongly implied through her behaviour and the fact that even her own father seems to think it. (And being a serial killer himself, he should have the necessary instincts to tell.)
What it all comes down to is that it's not really important if a friend really will be driving it up next weekend or if she dumped it somewhere. She definitly has something to hide about the car and it probably was involved in the murder.
My guess is that she actually did commit the murder, knew the car could be traced back to her and therefor lied to both her father and the police, telling each one the story that would get her out of more questions easiest.