I think your premise that this failure is not mitigate-able is incorrect. Some aspects are harder to mitigate, true - "has the attention of a Lord of Change" is tricky to remove from your character sheet. But it's not like Mathilde hasn't made enemies before. And while it's a bit more powerful than a Wisdom's Asp, it's not impossible to escape from.
I trust Boney to not make "Mathilde asks for advice on X situation" boring/pointless.
As I see it, the primary dangers from this are:
1 (very low probability) : Mathilde decides that the demon is right, goes for Everchosen.
2 (far higher probability): Mathilde ignores that a Lord of Change now is paying attention to her, fails to take steps to mitigate that, has Unfortunate Things happen as a result.
3 (not really known, probably between 1 and 2) : Something happens to suggest Mathilde is being demon-haunted and didn't report it, thus reducing trust in her further than if she did.