Anything enclosed in a metal shell is shielded by the shell from external electric fields. In other words, the external fields do not penetrate into the shell. This is why one of the safest places to be in a lighting storm is inside a car.
Consider two charges, -Q and +8Q. Which one feels the largest force?
As usual, Newton's Third Law tells us that the two charges feel equal-and-opposite charges.
A spherical conducting shell is now placed around the -Q charge so the -Q charge is at the center of a conducting shell. How does this change the force the -Q charge experiences?
The shell shields the -Q charge from the external field, so the -Q charge now feels no force.
Does the +8Q charge feel a force?
If the -Q charge is shifted toward the +8Q charge, but is still inside the conducting shell, what happens to the force experienced by each charge?
The -Q charge now feels a force because the situation is no longer symmetric. What happens inside the shell can not affect the situation outside the shell, however, so the force on the +8Q charge is unchanged.