How can you help?
Acknowledge your friend's feelings. Make sure to reassure your friend that her feelings are valid and common. Her condition is treatable and she can get better. Depression is an illness just like any other and there ought to be no guilt or shame attached.
Encourage your friend to see a medical professional. A recent report by the National Childbirth Trust suggests that 50% of mothers experience mental health difficulties during pregnancy or in the year following their child's birth. A further half of those don't receive the assistance they need, largely due to guilt, ignoring their own needs and even fear that their children will be taken away.
Assure her that it's okay to need help, that it doesn't mean she's failing. Her doctor may refer her to a local support group, recommend cognitive-behavioural therapy or prescribe a course of antidepressants. All of these things can be helpful. If she's anxious about attending appointments with or without the baby, can you spare the time to go with her? If not, the promise of a phone call later in the day may be enough to spur her on to attend.
The main thing you can do for your friend is be there. New parenthood can be isolating and this isolation only serves to exacerbate depression in a self-perpetuating spiral. Visit. Call. Send a card. Many new mothers are exhausted by night-time waking and struggle with day-to-day tasks, even without the added burden of PND. If you can spare the time, why not offer to look after the baby for an hour or two so she can do the simple things we so often take for granted? A short nap, a shower or space to take care of a few essential household jobs could help give her the breathing room to start her recovery.
Recovering from postnatal depression, like any other mental health issue, can take time. Just let her know you'll support her through the difficult patches and be there for as long as it takes.