Hello nlwaters. Nice to meet you. For the main question "If you knew someone who was about to drive home drunk, what would you do?" to be honest is a tough question.
First, I am no expert of law on this manner other than experience(s). The onus of liability to my understanding for a DUI injury or damages is the driver's if found to be intoxicated by any legal means defined, regarding adults. As to consequences that is a different matter.
Next, how much social responsibility & what could happen to me attempting to stop a person deemed drunk is the question. The drunk could say, "get out of my way!" Or, poke me in the nose?
From there escalation is the question and how much liability I have attempting to stop that person with social mores, folkways, and then both criminal and civil law. Next, is risk. Realizing any move I do, must be accompanied with accepting that responsibility.
Example of Life Experience(s). I use to drink socially at bars for whatever reason. I was diagnosed with diabetes. I still went to bars, could not drink because of medications. So, I became a designated driver at a few select bars.
Yes, I was single, I was bonded, I provided a free service, and had fun. I drank Free drinks from the bar - soda, water, coffee, etc, got free appetizers, and I liked green olives. So, those two primary bars said all I could eat of those on the house and they smiled too. And, I had to consider ethics and morality issues.
Without oddity, there was a clip board with a sheet stating they were "delivered to the door of their residence" safely. They were provided with a 8-1/2 x 11 sheet explaining how they got home, identified the service, identified me, I wrote down the bar they came from, the time of leaving the bar initialed by a bar employee, time of arrival, and provided all the legal mumbo-jumbo. A Taxi Service is different.
So, lesson is even though I provided a safe & free service for "drunks" being mainly people who either are irresponsible, simply got carried away having fun, or somewhere betwixt and between, well, I had to take responsibility for that free service. Fortunately I had a friend who explained these to me. I thought I would just drive people home. Remember they are generally speaking strangers.
Try this one on. You are a manager of an auto repair shop. The customers brakes are metal to metal & obviously unsafe. They refuse the minimum repair offered at cost saying no money. The law says they can leave. What do you do?