But I think the probability (that if Alice hadn't shouted me down then this guy would still be alive) is nontrivial.
I urge you to consider why you think this is true. You say that many other people warned this couple about the risks and all were blown off. If they had listened to your (or anyone's advice), there is a non-trivial chance of a different outcome. But was there a non-trivial chance you would persuade them differently?
(I'm also skeptical that your political disagreement had any effect on your persuasiveness regarding the danger of these drugs.)
I said I think other friends brought it up, but I'm not certain. What I meant is that while the chance of my talking them out of it, or at least talking them into some kinda moderation, is low, when the stakes are high even low probabilities are non-trivial in terms of expected value.
Why are you skeptical that political disagreements can break friendships? It can be worse - sometimes people get beaten up for supporting the wrong party.
Whether it's true in that case or not, it frequently happens that someone will start to act like they disagree with you about everything, because they happen to disagree about a few issues. And that definitely can cause them to do things that they otherwise wouldn't do, just because you suggest that doing them is a bad idea.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.