I've got the same problem.
I never got a quest from her to find the poison. The first time I spoke to her I asked the first question in the list, but then she wouldn't talk to me again. I did find the Duergar antitoxin on one of the Duergars that I killed and brought it back to her. This time she's lying down with only 2 health and is unresponsive. I can through the antitoxin next to her so it splashes on her, and then it shows she has the poison and the antitoxin in her, but there's no change. Just don't throw the antitoxin directly at her, or everyone will aggro on you.
Then I realized that the spell of lesser restoration will cure her of the poison, so it no longer shows as afflicting her. But when I try to talk to her all she does is call out for her mother.