what would you do?

Coming out of a gas station yesterday morning, I jumped in my truck and looked at the S10 next to me to see the driver pouring vodka into his drink. I'm not talking a splash, he was pouring. The worst part is this is the second time I have seen the same guy doing it. The first time I didn't believe what I was seeing. I passed it off as seeing things as I glanced at him as I walked by. This time I actually watched him pour. :( I don't want to get a guy in trouble, but I couldn't live with myself if he drove off and killed somebody. I got his plates and turned it in. Dispatched didn't think they could do anything because I didn't see him drink it nor did I see him intoxicated get behind the wheel. I don't know. I think I did the right thing but I still feel horrible at the same time. I kind of feel dirty...