Ahh, I can relate to not knowing what to do in this scenario, as the same thing happened to me. I'm not saying you should take this as advice as such, it's just an anecdote I have that happened 3 days ago now:
The duckling was moving about and trying to internally pip, so I went to bed content that there was movement in the egg. The next morning there was no movement whatsoever in the egg... I assumed it was resting so left it another day. After seeing it still not move a day later, I decided to try to assist with the hatch, I didn't get far into assisting when I realised that the gorgeous, fully formed duckling was dead. The cause was that the duckling had gotten shrink wrapped, and had membrane over it's mouth and nostrils. Absolutely devastated me as it was my first hatch. I then decided to assist the other two eggs hatch by making air holes. If I hadn't assisted then they would have died.
I personally don't see the harm in making a decent sized air hole for the chick/ducklings (EDIT: -when they have been trying to internally pip for a too long and are tiring), because to me it's just unnecessary death to leave them unassisted (though it seems to be a hot topic in the chicken/duck-keeping community). What I did in your scenario was make a small air hole, then a larger one if I didn't see any movement or a beak poking out the hole... But as I said, I have only hatched eggs once, so I am very inexperienced