A sorcerer in our group is quite fond of using polymorph to change herself into a giant ape (MM page323). At one point in our game last night she managed to grapple a Bheur (Winter Hag), that was riding a Nightmare (flying evil Pegasus). And during the fight the mount was killed sending them all into a 1200ft nose dive which did 120d6, (311dmg) to her polymorphed Ape form, which was it turned out exactly 3 points from killing her instantly with the massive damage rule (PHB page197).

An argument about the damage rule ensued, which I felt was squared away with simple math, but it then lead directly to another argument over the fact that she got to remain in her Giant Ape form because she still had the spell Death Ward active on her sheet. This might have just been simple confusion over how much time had passed after their long rest, as in her case she had Death Ward active for 16 total hrs, not 8, because of the divine sorcerer's "Extended Spell" metamagic (PHB page102).

The argument then CONTINUED to the entire table's near complete exasperation, as it became a debate over whether, or not, she should be reverted back to her original Sorceress form because she hit 0hp (which triggered the ward), or if she never actually hit 0hp because ward keeps you at 1hp.... Ugh

I made a ruling at the table that yes, she's going to continue on as a giant ape at 1hp despite taking over three hundred damage, regardless of how "ridiculous" the other players found it to be because those are the rules and also: "Because: MAGIC". But the RAW question that's now eating at me is: Does the spell Death Ward keep you at 1hp after you technically hit 0hp, or do you just never go below 1hp?

The Sorcerer retains their Giant Ape form with 1 HP

Death ward specifies the following

The first time the target would drop to 0 hit points as a result of taking damage, the target instead drops to 1 hit point, and the spell ends.

Emphasis mine. The point here being, the Sorcerer was the target of the spell. Their hit points were replaced with that of their new form, however Polymorph does not remove benefits of spells cast on the previous form.

This means their new hit point pool never drops to 0 hit points.

Ape at 1 HP

Death ward is still in effect

Since the ape is still a valid target for death ward, the spell is still in effect. If the ape were not a valid target for death ward, it would be suppressed.

Would Drop

The first time the target would drop to 0 hit points as a result of taking damage, the target instead drops to 1 hit point, and the spell ends.

"Would" is the operative word here. The meaning is expressing the future tense. Specifically, in the future where the damage drops the target to 0 hp. This spell prevents that future entirely as is indicated by "instead."

Restating this for this case: "If the damage would drop the ape to 0 HP, the ape instead drops to 1 hp and the spell ends."

Death Ward would prevent dropping to 0 HP

Death ward (PHB, pg. 230) says:

The first time the target would drop to 0 hit points as a result of taking damage, the target instead drops to 1 hit point and the spell ends.

Therefore, since the Giant Ape form of the Sorceress never hit 0 HP, I would understand that to mean that the polymorph spell does not end since that form never hit 0 HP.

since the Spell discription states "if it WOULD X it INSTEAD Y" i would say she never actually drops to 0.

"The first time the target would drop to 0 hit points as a result of taking damage, the target instead drops to 1 hit point, and the spell ends."

One might Argue though that loosing Polymorph does not qualify as dying even if she ""would drop to 0 hp""