If you don't want to call them, send them a detailed letter and don't sign it, trust me, they'll follow up. They don't care who clues them in, just do it. Every DUI is a potential homicide, after all. That's one crime that gets zero sympathy from me, I can understand some crimes, but there is zero reason for anyone to drive drunk - get a ride, get a cab, sleep on a buddy's couch, doesn't matter. No one gets drunk accidentally.