I would not consider it abnormal. Sometimes people just need to vent and they rather not talk to someone they know. You gave her a helpful ear and that was kind of you. People often start talking to me about personal issues, as strangers. As long as it does not get too personal, perhaps it is more helpful to the person than the listener can imagine.
@nedspeaks, I agree, it might even be safer for her to share with a stranger that way the story doesn't get back to the family.