When a player joins a coop game, enemies get a health buff.
(As detailed in <http://gaming.stackexchange.com/questions/63433/what-is-the-maximum-party-size-and-the-corresponding-monster-buffs>)

But when does this buff apply?
Let's say I'm fighting a boss and he has 1000 total hp, and I downed it to 50% hp, so he has 500 hp left. If a new player suddently joins the game, the remaining 500 hp for the boss get immediately increased? or it stays the same for enemies already spawned?