What is cd of marijuana? Did you read Carroll v. US. Here's a link:

Carroll v. U.S., 267 U.S. 132 (1924)

I haven't read it in a while but I still know the jist of it. You have more rights in your home than you do in your car because a car is mobile and evidence can be destroyed or moved and this is the case that helped set the caselaw. To perform a warrantless search you have to have probable cause and an exception to warrantless search. Those exceptions are consent, exigent circumstances, plain view, incident to arrest, terry pat, inventory, booking, or administrative. I have to warn you, this cop I know always blasts me when I explain it because he likes to split hairs and I know what I want to say but sometimes it doesn't come out right so I'll try my best. With a car exigent circumstances exist because evidence could be lost while trying to obtain the warrant. Another exigent circumstance would be if someone was in danger. An example of that might be if a cop came to your house and wanted to question you and when he went to ring the doorbell, there was fresh blood all over the front door. Consent is obvious, plain view is obvious, terry pat is when an officer frisks you to check for weapons for his safety even though he hasn't arrested you and he finds something. Terry pat is more complex than that because I think there are rules to not turning pockets inside out and more things like that. I should stop trying to explain the other exceptions because I will say it wrong. They also can get around a search warrant requirement for a car by towing it and taking inventory before towing it so you don't claim something was stolen.

I'm basically trying to say you have to have probable cause and an exception to the search warrant requirement to search your car but because it is a car you have far less rights than your house so it is easy for them to get around this. I'm not saying the search was bad but it is going to be harder to challenge than a house. Plus they can argue inevitable discovery. They might have done something wrong but inevitably they would have found it even though at some point they screwed up. Conversely there is "fruit of the poisonous tree" which means that had they not screwed up they would have never figured things out so everything discovered is "poisonous" and off limits.

I should get Birdgirl to rewrite all that because she is quite eloquent in her writing.

If your friend had pot in the car or a pipe, I bet the officer smelled it. People who don't smoke weed, especially police officers, can smell it very easily. If the cops says he smelled it, your friend will be fighting an uphill battle. So he might want to go back to whether the stop was valid or not.