[SPOILERS] Stormblood Ending

[Daralii]

But the question remains: when did he learn? Until we know that, it's not fair to blame anyone but her family, who definitely knew.

Gosetsu was also a retainer to the king of Doma, so he was in a fairly privileged position as far as information was concerned. The death of a noble would have undoubtedly made its way to the castle.