being that she was Robert’s betrothed and how tight Ned and Robert were, seems likely it was done in secret. And even if Ned DID know, he may have fought to get her back out of Ned’s belief in honoring the pledge that had been made

It would seem exceedingly odd to deny Tormund and Beric a meaningful heroic death on the frozen lake (flaming sword holding off an onslaught of dead guys, or Tormund just Tomunding away to buy the others time, or something) just to have them die unceremoniously in a collapsing wall. Two possibilities I see are either…

Or the horn in the books isn’t the genuine article