1. If she's in denial, maybe it's not time yet to talk about it nor validate her feelings about it. You could hint at it to find out, but if she's not ready then she's not ready. No need to be a caretaker for her emotions. You can't be responsible for reading someone's mind.
2. Probably when you care more about the person than yourself. Another line to watch out for is supporting vs enabling,
see here.
I saw from your post history that you know about Dr. Fjelstad. Have you read her book, Stop Caretaking the Borderline or Narcissist? I think it does a good job describing lots of these. Reading it helped me a lot.