Hi!
I had this dilemma on my last hatch with two eggs that zipped good at first and then nothing. I chose not to assist because I felt it best to give them time and to not risk hurting them...you can kill chicks pretty easy by "helping" and I'd regret it if that happened. Usually patience with hatching pays off for me.
I also based my decision on the thought process that if they are too weak to fully hatch, they will not make strong, viable chicks to raise.
After about 20 hours when the exposed membrane started to dry, I checked for signs of life and they were both dead. After opening the shells, it appeared that the yolks hadn't fully absorbed so I probably would have killed them anyway if I had tried to help. It just wasn't meant to be for them.
It's a big decision. You can check for signs of life before taking any action (if any). I think you can hold the shell to your ear and tap it to get them to peep if they are alive inside...also watch for movement where the shell is zipped.
Best of luck!