When a player joins a coop game, enemies get a health buff. (As detailed in 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?