Will you probation be violated if you catch another charge?

A friend of mine is on probation for drug and firearm charges. He did 4 1/2 years in prison for these charges and upon his release from prison they put him on 5 years supervised probation. He just got out of prison in September of last year. Last month he was driving a friends car because she was too intoxicated to drive herself. He was simply going to drop her and the car off and take a taxi back home. He was not intoxicated or under the influence of any drugs at the time of operating his friends vehicle. The female whom which the car belongs to, had beat up an informant earlier that day and the police department was already looking for her vehicle. She also has 2 lbs of marijuana under the driver seat of the car. The police pulled them over. My friend had no idea of the earlier incident with the informant and had no knowledge of the drugs in the car. When the police pulled them over while he was driving, they charged him with possession of CDS (marijuana) with intent to distribute. They have also added an after prior conviction in to this charge. My question is, will the state automatically violate his probation or is there a way he can fight this charge since he had no knowledge of the situation? I need advice. Is he looking at going back to prison? Please help. Thank you.