I think the thing that would most help your friend bond with her baby is effective treatment for her depression.
I had pretty bad PPD with my daughter, aggravated by the separation involved in her NICU stay, and the first serious symptom was the feeling that I was not bonded to, and could not bond with, my daughter. Breastfeeding did not make this problem go away.
One of the things I would be aware of, on the basis of my experience, is that when the whole world feels grey and dreary, you are not going to look at a baby and feel all the joy and love in the world. You probably want to pull the covers over your head and sleep forever. The baby looks more like a little skin bag of screaming need than like a child to delight in. And it's hard to be really observant and notice the signs that the *child* adores you when you're in this fog. It's really important to keep in mind that the way the parent feels about the child is NOT the same way the child feels about the parent - your friend's baby may think her mama hung the moon.
Since DD was early, we qualified for EI, which turned out to be (for us) a therapist who came to the house once a week and fussed over the baby while I sat down with nothing in my hands and drank a cup of tea. It sounds like nothing, I know, but that woman managed to notice more details about DD than I was capable of, and was therefore able to point out that she was growing and changing and that she seemed to like me. Our pediatrician was also great with this kind of stuff, just noticing the things that babies do as they grow and develop relationships with their parents, and commenting on them. You do not have to be a therapist to ask if you can hold the baby, and tell her mother how alert and responsive that baby is.
As much as this stuff helped me feel better, talk therapy and medication were also key (and remain vital) for my mental health. If your friend is on medication and still feels this bad, she possibly needs to ask a psychiatrist to reevaluate her meds.