What do you think of the police's advice to my friend?

Your friend had done the right thing. Being alone in a deserted place there was no where your friend could overcome these men if they had alighted from her car. The police was just trying to make this ( lack of information ) an excuse not to open a file and investigate the case. What happened to your friend was not an accident, it was planned by those three men to rob or probably your friend, therefore most likely the car plate was a fake one. In situation like that, we have to think of our safety first, whether the men would be caught was not important at that point of time. Malaysian police cannot be trusted, even with sufficient information, they would not look into cases like this seriously.