Surely this only makes sense as a concern in terms of challenge base play? All play changes the narrative and the current state of the fiction. I hope you'd agree that the runes could possibly be helpful for one of many reasons - creation of fiction that makes them so is not implausible.
If the play isn't challenge based, then it doesn't matter at all whether the runes provide a benefit to the character - it's simply an advancement of the unfolding history of their actions and the world's reactions. Issues of fairness and mechanical benefit don't matter if we are simply interested in following that history - strokes of what appear to be great good luck are not uncommon in genre fiction.
From what
@pemerton has described of this, the player is making good faith action declarations that follow the rules of the game they are playing. No cheating is involved, in so far as there is a mechanical challenge, the player is using their characters capabilities, leveraging an aspect of the environment in accordance with the rules for doing so in order to overcome the peril the character faces. I can see how this might appear to be "cheating", but I suspect it's not possible to get this kind of benefit without a check - so we might postulate a situation where a D&D player leverages some aspect of the environment to overcome a challenge without a check purely by negotiation with the DM (in the classic door spikes and 10 foot poles mode) which would in turn appear to be "cheating" (that is, in this case obtaining a mechanical advantage without a check) to players of
@pemerton 's game.