I heard Trek Central suggest this, but I'm not so sure.

If we're looking at a Prime Directive violation, we're looking at the interference with the social development part, or on a more granular level, interfering with the internal affairs of a civilization.

Sure, Bragh was a high ranking Klingon being part of the Oversight Council, but the death of Bragh was between Ma'ah and Bragh. Boims and Mariner participated in the Rite of J'ethurgh, but that wasn't interfering in Klingon affairs, no more than Picard participating as Worf's cha'DIch was. Technically, Ma'ah accepted them as part of his quv beq, so they were invited in.

And at the end, as far as Boims and Mariner is concerned, the Rite was over and completed - Bragh being a sore loser and the subsequent fight had nothing to do with them and they didn't participate in it - only witnessed it. Nor was the fight a foreseeable consequence of Mariner trying to get Ma'ah reinstated so she could get a Klingon Captain to assist in her mission, and especially not Bragh's death, which was only because he literally stabbed Ma'ah in the back after yielding (by granting Ma'ah his captaincy back).

So I really don't see the problem here. At worst they were bystanders to the death.