What is a Muslim woman permitted to reveal in front of a non-Muslim woman, and what are the limits of that?

All praise is due to Allah.

A woman may reveal to another woman, whether Muslim or disbeliever, what is above the navel and below the knee. As for what is between the navel and the knee, it is awrah (private parts) and a woman should never see it from another woman. So, a woman may see from another woman her chest, head, and leg, just as a man may see these parts from another man. The opinion that a believing woman should not uncover in front of a disbelieving woman is weak, because Jewish and pagan women used to visit the wives of the Prophet (peace and blessings be upon him), and it has not been preserved that they would veil themselves from them.