As long as they have access to the atmosphere OUTSIDE of the egg I would let them go for a bit longer. Make sure your humidity is good and the membrane does not dry up. This is the thing to look for before you go to help out.
As you know, Guinea egg shells are harder than chicken eggs. The chick may just be taking a break and absorbing its egg yolk sac before continuing on.
If you do intervene, make SURE you do not go below the eggs air sac line at first. Let the chick struggle first and be sure the egg is on its side if you take off the shell.
If you see ANY blood, stop immediately. Sometimes you have to "unfold" the chick from the egg. You will know what I mean if you see it.
Good Luck, let us know what happens.