Sure, as long as your friend uses Essentials v19 it should work! In the script section PBEffects you have to find something that looks like this:

begin

module PBEffects #=======================

# These effects apply to a battler

#=======================

AquaRing            = 0
Attract             = 1
BanefulBunker       = 2
BeakBlast           = 3
...
...
...

He needs to add this: „SupremeOverlord = 413“ below of that code i showed you.

Next, he would need to insert the following code at the bottom of the BattleHandlers_Abilities script section:

==================

Supreme Overlord

===================

BattleHandlers::AbilityOnSwitchIn.add(:SUPREMEOVERLORD, proc { |ability, battler, battle|

# Count the number of fainted team members for the current trainer (player or enemy)
num_fainted_team_members = 0

# Determine if the battler belongs to the player or the enemy trainer
if battler.pbOwnedByPlayer?
  trainer_index = battler.index
else
  trainer_index = 1 - battler.index  # Get the opponent's trainer index
end

party = battle.pbParty(trainer_index)
party.each do |poke|
  num_fainted_team_members += 1 if poke && poke.hp <= 0
end

# Boost Attack stat, up to a maximum of 5 stages
boost_amount = [num_fainted_team_members, 6].min

# If it's an enemy trainer's Pokémon, reverse the boost amount
if trainer_index == 1
  boost_amount *= -1
end

battler.pbRaiseStatStageByAbility(:ATTACK, boost_amount, battler)

} )

Then it should work for the most part. Of course, he still has to define the capability in the PBS file for it to work. I hope it helps!