What is the ruling on breaking an oath after revealing a secret I swore to keep, in order to warn someone whose identity had been impersonated? And am I required to offer expiation?

You are not obligated to fulfill your oath to this person not to tell anyone. You may break it and inform your friend, because concealing this would harm your friend, whose identity this person impersonated and whose secrets he interfered with. You must offer expiation for breaking an oath. However, if you swore based on the belief that it was your friend, and had you known the truth, you would not have sworn, then expiation is not obligatory upon you when you break the oath.