Over the years there has been problems with the Municipal traffic cops. especially at the south end of HMO on that bypass. It is hard to do but you must stand your ground. Even take a photo of the cops. Tell him you want to see the chief of police. There is no place to park an RV at the police station or the court house. They will just give up. Never speak Spanish! If you have a car number or names I would contact Sonora Tourism. They do not carry breathalyzers so cannot charge you with a DUI. You can demand to see the radar gun. But sometimes it is stuck on a high speed! The last time I was involved the poster sent me a photo of the cop. I sent it to my contact @ Sonora tourism ( alas he died a few years ago) and the cop was fired. He cried that he was a family man and they suspended him for awhile. If it happens at night sometimes the district chief is part of the scam. But during the day it is just the 2 creeps. Very frustrating. I hope your friend gets over his experience.
Moisheh