"Friendly hits" feature should be used only for player's followers
In OpenMW, enemies call out friendly fire when they're being hit by one of their own (which might or might not be intended), but also when they're being hit by you (definitely not intended) seemingly after they've been hit by friendly fire.
This behaviour can easily be observed with Trerayna Dalen and her group in front of Tel Branora, or Farvyn Oreyn and his bodyguards during Malacath's quest.
When you aggro one of their group, everyone will turn hostile and they will damage each other with AoE spells if you stand close enough, which will lead to them calling out friendly fire. But even after they've run out of spells, some will call out friendly fire when you hit them, possibly the ones who received actual friendly fire before.
I haven't done a lot of testing with this, but I think in Morrowind enemies do not call out friendly fire at all, even when it would technically be appropriate.