"drifting" into another lane is an offense justifying detention IF it is unsafe. Since your friend apparently believes it was not unsafe, there needs to be a fight about it in the form of a motion to suppress. Hopefully, the cop's video will make that much easier to evaluate by attorneys and judge. In any event, the traffic violation apparently did not occur in the county of the cop's authority, so that could provide a possible defense to the initial detention, so your attorney needs to look at that very carefully also. The DWI offense occurred in both counties, so a conviction is probable IF the state is able to justify the initial detention, which seems to be a good fight to pursue. Get a really aggressive DWI attorney who wins motions to suppress regularly-and no one else to defend you.
I'm giving you the code sections below. The offense for which the officer stopped your buddy arguably (and by the time the cop confers with the prosecution will have) started in the original county - so that is the venue (county of prosecution). Cop will likely say that there was not a lot of traffic so buddy was not a terrible danger & cop wanted to watch the driving to give due consideration (or whatever....)
If the cop lies about the county, then hopefully there is a video to show it. OR, buddy's lawyer can get the tow slip which should list the location....
"Drifting" is one of those ever evolving questions in probable cause. Yesterday it wasn't sufficient; today it is kind of thing. I'll lay odds there will be more to it than that when push comes to shove (a trial.)
Buddy needs a very, very good lawyer to try to stop the DWI conviction train from leaving the station.
TITLE 1. CODE OF CRIMINAL PROCEDURE CHAPTER 14. ARREST WITHOUT WARRANT
Art. 14.01. OFFENSE WITHIN VIEW. (a) A peace officer or any other person, may, without a warrant, arrest an offender when the offense is committed in his presence or within his view, if the offense is one classed as a felony or as an offense against the public peace.
(b) A peace officer may arrest an offender without a warrant for any offense committed in his presence or within his view.
(d) A peace officer who is outside his jurisdiction may arrest, without warrant, a person who commits an offense within the officer's presence or view, if the offense is a felony, a violation of Chapter 42 or 49, Penal Code, or a breach of the peace. A peace officer making an arrest under this subsection shall, as soon as practicable after making the arrest, notify a law enforcement agency having jurisdiction where the arrest was made. The law enforcement agency shall then take custody of the person committing the offense and take the person before a magistrate in compliance with Article 14.06 of this code.
NOTE - Penal Code 49 is the DWI section
Subtitle C. Rules of the Road
Chapter 541. Definitions
Chapter 542. General Provisions
Chapter 543. Arrest and Prosecution of Violators
Chapter 544. Traffic Signs, Signals, and Markings
Chapter 545. Operation and Movement of Vehicles
Chapter 546. Operation of Authorized Emergency Vehicles and Certain Other Vehicles
Chapter 547. Vehicle Equipment
Chapter 548. Compulsory Inspection of Vehicles
Chapter 550. Accidents and Accident Reports
Chapter 551. Operation of Bicycles, Mopeds, and Play Vehicles
Chapter 552. Pedestrians
Chapter 553. Enactment and Enforcement of Certain Traffic Laws in Certain Municipalities
Chapter 600. Miscellaneous Provisions
A defense attorney would not file a motion to dismiss. That would tell the prosecution that they had problem with one of their elements and put the case in the right county. Not smart (unless two years has gone by)
If the officer saw DWI in two counties, he can charge in either.
The drifting is a real issue. That might not be cause for the stop. Depends what the cop says on the stand, i.e. "Now that you mention it... he did fail to stop at a stop sign..."