You're a little vague on the exact effect. Is the effect based on how much the target has healed? How much the user has healed? What I can tell you is that to create such an effect, you'll need some way to monitor the amount of healing that's occurred in that battle. If you have Yanfly's Auto Passive States, Skill Core, Damage Core, and Buffs & States Core, such an effect may be possible.
http://yanfly.moe/2015/12/25/yep-50-buffs-states-core/
http://yanfly.moe/2015/11/07/yep-25-damage-core/
http://yanfly.moe/2015/10/17/yep-13-auto-passive-states/
http://yanfly.moe/2015/10/13/yep-8-skill-core/
The video below is for an Undo effect, which has a few common elements with what you're trying to do.
Put altogether, we can do something like this:
For the state:
<Custom Battle Effect>
user._healedAmount = 0;
</Custom Battle Effect>
<Custom Respond Effect>
if (this.isHpEffect() && value < 0) {
target._healedAmount += value;
}
</Custom Respond Effect>
For the skill:
<Damage Formula>
if (target._healedAmount !== undefined) {
value = target._healedAmount;
} else {
value = 0;
}
</Damage Formula>
Try that, but make sure that in the Auto Passives plugin, set the state under Global Passives. That way, everyone automatically gains the state.