My 20 year old friend was pulled for failing to flash a turn signal. after, he was asked if he had consumed any alcohol.

Probably not. If your friend was being arrested, the police have the right to search the car based on the arrest. I am not sure why they went through the charade of asking to search when they always had the right to do it anyway. Usually, the police only ask if they could not do a search without the suspect's permission. Your friend, though, would be wise to hire a lawyer who can consider all of the aspects of his case in detail. In my experience, it is frequently possible to avoid all conviction in circumstance like this one, assuming that your friend has a clean record.

Legal issues regarding searching vehicles can be relatively complicated. See: Arizona v. Gant, 556 U.S. 332 (2009), was a United States Supreme Court decision which held that the Fourth Amendment to the United States Constitution requires law enforcement officers to demonstrate an actual and continuing threat to their safety posed by an arrestee, or a need to preserve evidence related to the crime of arrest from tampering by the arrestee, in order to justify a warrantless vehicular search incident to arrest conducted after the vehicle's recent occupants have been arrested and secured. In an opinion delivered by Justice Stevens, the Supreme Court held that police may search the passenger compartment of a vehicle incident to a recent occupant’s arrest only if it is reasonable to believe that the arrestee might access the vehicle at the time of the search or that the vehicle contains evidence of the offense of arrest. Justice Scalia wrote a concurring opinion, "In my view we should simply abandon the Belton-Thornton charade of officer safety and overrule those cases. I would hold that a vehicle search incident to arrest is ipso facto 'reasonable' only when the object of the search is evidence of the crime for which the arrest was made, or of another crime that the officer has probable cause to believe occurred." Justice Alito wrote a dissent joined by Chief Justice Roberts, Justice Kennedy, and Justice Breyer in part, saying that the court had effectively overruled New York v. Belton and Thornton v. United States, 541 U. S. 615 (2004).

As Bill Powers wrote, the issues are complicated and Arizona v. Gant is relevant here. However Gant does say that police can search the vehicle if they have a reasonable belief that evidence related to the crime might be found therein. In other words, the ruling does not strictly hold that the search is only valid if it meets one of the twin purposes of Chimel/Belton - Officer Safety or Possible Destruction of Evidence. The facts in Gant were that police arrested the defendant on an outstanding Driving While License Revoked warrant, and secured him, and then searched the vehicle. In the facts as described in this question here, a court might find police might have a reasonable belief in finding incriminating evidence in the car related to a DWI - beer bottles etc. Again, search and seizure law is very fact intensive. You might have a good defense, but it's not exactly clear from Gant that you do.