Provided the 'rebel' councillor is careful to tell the truth and to have supporting evidence, I don't see any risk of action by the Monitoring Officer.
Although it no longer applies, the standards regime that applied prior to recent changes actually stated that a councillor who was aware of a breach of the standards was obliged to report the matter, and would themselves be in breach of the standards if they failed to do so. Although this is no longer enforceable, I would think that it remains a reasonable principle for action.