R.v. Soroush et al, 2022 MBCA 84, at paragraphs 40, 56 and 57:
In summary, the rule at its core is a principle of trial fairness. If one party intends to impugn an opposing witness’s credibility, whether by leading contradictory evidence or by challenging it in closing arguments, that party is expected to confront the witness in cross-examination so that the witness is not “ambush[ed]” and has the opportunity to explain.
…a remedy for a breach of the rule is not restricted only to the party whose witness is impeached. In my view, the rule should be applied consistently with its fairness origins, so that unfairness to any opposing or adverse party or co-accused is also captured. I find support for this in the authorities indicating that the rule seeks to prevent unfairness not just to the parties, but to the witness and to the trier of fact. Finally, I note that the rule places a dutyon the party who intends to impeach a witness to give fair warning to the witness that their evidence is not accepted. This suggests that the focus should be on whether the impeaching party has met this duty rather than on which party objects to the breach of that duty.
For these reasons, it is my view that the rule can be engaged where the Crown impugns the evidence of a co-accused on a point of substance for the accused in closing argument rather than making the challenge in cross-examination. Clearly, the question of whether the rule is actually breached will depend upon all the circumstances and the fairness concerns present in any given case.