incorrect, as i had 3 heralds ready to spawn, told a friend and went afk.
he joined the game, had all 3 spawn and killed them
I don't remember when I am in a public game to ever spawn another active player's herald (I have found 3 latent sunder charms so I have hunted a lot).
I am not sure if because you left the game the ires were "transfered" to the other player. I know that if I leave the game and return with the same character I still have the ire so it might be that I can rejoin with another character and still be able to spawn based on what you said.