When to assist? Excellent question that I find impossible to answer. We all have different opinions. Some people are full hands on, opening the incubator, candling, poking holes, and who knows what else. I'm more hands off, with the opinion that the more I interfere the more likely I am to cause harm.
The chick has to do a lot of things to hatch like dry up blood vessels in the membrane around it and absorb the yolk. It does other things also but these are two that are more noticeable. Some chicks do a lot of these things between internal pip and external pip, others wait to do more after external pip so zip can seem to take a long time. I don't believe you can see that all that well by candling the egg, it's just a dark blob. You can often tell if it has internal pipped by seeing the beak in the air cell but what does that tell you as far as actions to take?
In your situation I'd wait. If that chick cannot external pip on its own it's probably not going to make it anyway. Others would have you poking an air hole.
Good luck!
