What did Nelly do when Catherine was suffering in fever?

Nelly is angry at Catherine. She thinks Catherine is acting the part of a drama queen and pretending to be sick to manipulate Edgar. However, Nelly is quite wrong. Catherine genuinely is very ill.
Catherine, who is pregnant, becomes sick because Edgar tries to force her to choose between himself and Heathcliff. His demand throws her into a fit, but Nellie advises Edgar to ignore it. She tells him that Catherine had planned ahead of time to throw a "fit of frenzy."
When Catherine runs to her room, Nellie persists in not taking her illness seriously. She doesn't tell Edgar Catherine is ill. When he doesn't come to visit her, Nellie says he is caught up in his books, which Catherine interprets to mean he doesn't care about her. This makes her sicker. Nellie states:

I could not get rid of the notion that she acted a part of her disorder.

As Catherine becomes increasingly sick and disoriented, Nellie still doesn't tell Edgar. When he happens to burst in and sees how far gone Catherine is, he is very angry. Nellie, frightened that she will get into trouble, lies, and says Catherine

would admit none of us till this evening, and so we couldn’t inform you of her state, as we were not aware of it ourselves; but it is nothing ...

Of course, this is entirely untrue. Nellie had every opportunity to inform Edgar, and Catherine wanted her to, but Nellie took it on herself to let Catherine decline without telling Edgar. It's hard to avoid the conclusion that she is complicit in Catherine's death.