Is informing a relative who was harmed by a friend about what she did considered divulging her secret and a sin?

You must keep the secret entrusted to you, for the Prophet, peace and blessings be upon him, said: "If a man narrates a حديث (statement/story) and then turns his head, it is an amanah (trust)."

However, if this secret contains harm that will befall another person, then its occurrence must be prevented as much as possible, even if it means informing the person concerned to warn them.

But if the harm has already occurred and concluded, then it is not permissible to disclose it, because it is an amanah (trust) and to avert fitnah (strife/discord). You must advise your friend to repent and seek forgiveness from those she has wronged.