Gui uses local script to function, but it doesn't display the same value for everyone else?

I have health gui for a boss in my game. It goes up and down depending on the health. When I had a friend play the game with me, we found out that the boss's health would only change the gui if you were the one who shot it. If someone else shot it, it wouldn't change at all.

It is a local script, and I don't know how to change it into a script.

Here is my script:

wait (.1)
local visible = script.Parent.Parent.Parent.Visible
local bframe = script.Parent.Parent.Parent.Parent
while true do
	local hp = game.Workspace.Dummy.Humanoid.Health/500
	script.Parent:TweenSize(UDim2.new(hp,0,1,0),Enum.EasingDirection.Out,Enum.EasingStyle.Quad,0.15)
	wait (0.1)
	if hp < 0.01 or hp == 0 then
		bframe.BaseFrame.Visible = false
		wait (7)
		bframe.BaseFrame.Visible = true
		script.Parent:TweenSize(UDim2.new(0,647,0,70))
		script.Parent:TweenSize(UDim2.new(hp,0,1,0),Enum.EasingDirection.Out,Enum.EasingStyle.Quad,0.15)
		local d = game.Workspace:WaitForChild("Dummy")
		local hp = d.Humanoid.Health/300
		script.Parent:TweenSize(UDim2.new(hp,0,1,0),Enum.EasingDirection.Out,Enum.EasingStyle.Quad,0.15)
	end
	wait (.2)
end