I’m carrying a level 30 through ICC gearing her up for Herald of the Titans. I killed Deathbringer Saurfang and the chest contained the legs she needed. However, only I could loot the chest, and the loot I took was not tradable.
Chests that do this always do it. Only the highest level member of the party can loot the chest ever, and the loot is never tradable. So going back again next week we will have the same issue.
This has been a problem with various chests since level scaling was implemented during Legion.
Edit to add: the Four Horsemen chest in Naxxramas is the same. Only the highest level party member can loot it, and the gear is not tradable.
Did your fellow player tag the boss? Sometimes it might take several tags for it to register. I know I had problems with the Faction champs one when I was helping my brother gear up a toon for Herald (and blizzard said get rekted as a response) even after waiting like 5 minutes before I killed the last enemy in that boss encounter.
Some of these fail even in normal queued dungeons. Some only fail when you run in. Most chests work properly and are lootable by the entire team.
These borked chests are set up improperly. It wouldn’t be an issue if the loot inside was tradeable, but it’s not. The target character shouldn’t have to solo a raid boss to get their loot.