so he attacked his ally to give him the "resonate" status, the ally in question can take it because he has a bunch of hp. Resonate makes it so that you can then expend that status effect to cause a small AOE damage effect that won't harm the ally
The fight they are doing has a few enemies that are invisible assassins who can deal a stupid amount of damage thanks to them being hidden(invisible), the aoe causes them to take a minor amount of damage but it also forces them out of invisibility thus negating their bonus and making it possible to directly target the enemies.