I find it extremely difficult to find any justification for how Elaine treated the main character. Even if she wanted to be nice, she still chose to be mean. Best case scenario is that she honestly believed that being tough on the kid would help him out later on in life, which is what the father would've been demanding of her. Worst case scenario is that she is the gold digger that the father claimed she was, and she is (once again) only after the money.
She's really suspicious, not only with her sudden change in character, but the fact that her attempt to make amends with the main character was by giving him a car that was already legally his. So I'm leaning towards her being a gold digger. However, I wouldn't mind being wrong since she's smoking hot.