Someones we all need a vent. If you hold someone the space to say what they need to — and truly know they've been heard, that opens up space for them to hear.
Well that is something I can 100% relate to, but it's not the situation I was thinking of.
A typical situation would be someone trying to deflect blame by ensuring they have talked to someone else before making a mistake, just so they won't take 100% of the fallout.
That sounds like not your problem. If A deflects blame to B, to escape C's judgement, then B can make their case to C separately. There's no reason to prevent A from having a conversation from B at all.
Frustrating sometimes? Definitely. But it works.