If she does not think that she has a problem , then it is quite tricky to get anything done.
Is her Health Visitor calling regularly? Might be a thought for husband to give the HV a call to put them in the picture so they can drop round more frequently and ask appropriate questions when they do.
Or husband can go to the GP himself to express concern , then the GP can pay a 'courtesy visit ' to the new Mum and babe and take things from there.
In terms of helping directly....offers to help,out in the house or with the children might help, but in truth if she does have PND then she almost certainly needs medication to get over it.
Like all deprssive illness it will eventually go whether it is treated or not, but it can take a heck of a long time to go if it is untreated.