But it hardly is of much use in morally ambiguous situations. When a partner of yours (I'd suggest even imagine it's your best friend to make the thing more vivid) is a real criminal inflicting real harm to real innocent people (rather than smoking weed in a wrong state) you have to choose whom to betray. There is no single right solution here. One has to decide for himself. It is just an act of will.
I don't mean he should not have asked, I just share the way I view such a problem.