25+ years ago, long before cell phones, I followed an obvious drunk for a good 5 or 6 miles (my thinking was that if he hit something/someone or ran into the ditch, I'd at least be able to help).....all the way to the parking lot of another bar!!
Nowadays, I'd drop a dime on someone swerving like that, in a second. Chances are they're drunk, and you might be saving some innocent life. Could also be they're having a seizure or are suffering from diabetic shock, and you might save the driver's life.