A.K.
I went through post partum with two of my boys. If she hasn't already, your friend needs to get to the doctor and get treatment. Post partum is a chemical imbalance in the body and it won't fix its self right away. There is no reason to be unhappy when there is help.
As a friend, being there for her is the biggest help. Whether by phone, visits or getting her out of the house when you have a day off.
Hope this helps!
A.