Tradition One suggests that we do not attack the other person or their planned course of action, because we need to maintain unity if anything is going to be achieved.
Tradition Four means it's your business; they're not autonomous because you're collaborating.
Tradition Two means you sort this out together. Avoid email. God speaks far more effectively through a group conscience held in person.
Tradition Five suggests you establish what your primary purpose is: raise the level of the discussion to the ultimate objective that you can agree you want to achieve.
Once you have an ultimate objective identified, look at what the available options are, including the irrational course of action suggested.
Examine how each of those is most likely to achieve the ultimate objective, and at what cost.
Do not appeal to pure logic and reason, as these are fallible resources:
... and ...
Instead, appeal to experience (the Big Book refers to experience as personal adventures). It's much more convincing.
If not, and the irrational course of action is going to be persisted in, there are two options. Either you can bow out or you can put up with it. The latter is the case if the person in question is a superior and can pull rank. Have your objections noted and aim to be as helpful as you can. You have done your bit.
Al-Anon comes into play, here. As with alcoholics, who need to hit rock bottom first, sometimes people need to pursue the wrong course before they will be convinced that it is the wrong course, and we need to not stand in the way. As with alcohol, experience is sometimes the great persuader.