I have been successful with intervening in such situations, as I'm one of those that just cannot leave a chick to struggle and die in the shell.
This is the process I use (which you may choose to use as well, or not, depending on how you feel about the subject):
Wash your hands with soap and water. Get some Q-Tips and a cup of hot water, plus a clean wash cloth. If you don't have nice, strong fingernails, get a pair of sewing snips or maybe fingernail clippers. Open the incubator and snatch out that egg, and wet the Q-Tip with the hot water. Pour some onto the washcloth and hold the egg with it. Daub the exposed membrane with the wet Q-Tip so it darkens again and becomes translucent. Start picking at the edges of the shell, just little bits at a time.
If you encounter blood, stop immediately and put the chick & egg back into the 'bator for a few hours.
If not, work around the edges of the shell until as much of it is removed as possible. Work fairly quickly because you don't want it to be out of the heated environment of the incubator for very long.
I tear the membrane gently to release the chick's upper body. Again, if you encounter blood, stop immediately.
I usually put the chick back in the 'bator with half of it still in the bottom of the shell and the membrane so IT has to kick its way out as if it would have done on its own, had you not assisted it. That lets it finish hatching. I think that's important to the hatching process. Especially since there may be some un-absorbed yolk still attached to the chick, in the bottom half of the egg shell. Let it "finish" for several hours in the 'bator.
Good luck in either case!
