Heals and buffs from other players is only from souls as far as I know.
The second you click attack, your party and the boss's damage are already calculated, that's why you can just click refresh icon on the bottom left to skip the entire turn animation or you can literally check log to see your total calculated damage for the turn before the animation is even over. You can even see if your party wipes or not.
So support heals and buffs will only apply before or after that and immediately when they use it.