It doesn't matter if it's your car or his, and it doesn't matter if you knew it was there. all parties are held accountable for substances in the car. Now, I don't see why he arrested either of you for a crumb. Around my area they make you throw it on the ground and step on it. But he shouldn't have had it in his car in the first place. I would get a lawyer. And not rely on my friends testimony. The court will not care for his confession. You should go ahead and consult with a lawyer and see what he thinks..