Found a Topic in 2022, and its closed, so:
Some auras still effect the other player as an attack, they take no damage, but they sometimes proc on hit effect on gear and kill the non-aura player very fast sometimes:
In the last 2 days we had this happen twice: (newest patch, all Add-Ons)
First I activated Guile and my friend had green shoulders that procced a lightning bolt on hit, I lost like 60% hp ever 2 seconds. Next day it worked like normal and I was fine.
Secondly yesterday I swapped my boots to an on hit proc (Hellion Greaves, blue lvl 50), and he had only 3 Buffs: Possession, Witchblade and Blood of Dreeg (only this one should effect me).
This time he triggered my boots proc.
After ~1 h (I switched my boots back to non proc boots because of that) I switched to the proc boots again and this time he did not proc my boots (we where in the same game, no re-host)
In both scenarios I was the host.