Ravingdork
Explorer
Your GM was right. The stat block says that the two jellies are identical and you do not divide hitpoints after they split. So if a hit brings the opchre jelly down to 49 hitpoints and it splits, then each of the jellies has 49 hitpoints after the split. The reason for this is that the jelly is an elite monster, so allowing it to act as two monsters after it is bloodied is the sort of thing that is supposed to happen. 25/25 would have been ridiculously easy to take down.
I actually ran this encounter for my players last night, so I know whereof I speak. They wiped up the ochre jelly quickly enough. No need to make things even easier.
It really, truly says that in the module? The monster manual oozes clearly indicate that the HPs are split between the halves.