I think the problem is she brought it to her organization's doctor that often does the ER admitting in situations like this and that doctor overruled her.

The NP could still have done it, but it would have been against two different doctors and the organization who employs her practices. The sad thing is that this probably happens often, this is just the first time someone sued over the death.

From what I know about hospitals the NP is supposed to defer to the doctors.

The NP did the right thing in a sense though - she went up the chain of command. The person up the chain, however, did the wrong thing.

edit to add: possibly the NP went up the chain of command precisely because the doctors there wanted veto power on hospital admissions. they didn't want pesky underlings getting in the way of whatever motivated their decisions.