hi there
a good friend of mine is also suffering from PND here are some things i have done with her:
1. when i cook meals for our freezer i make a couple and take them around to her as well. i make it like we will do this for each other, rather than that i am 'saving' her iykwim - i also love cooking so i position it that way..
2. now that our babies eat solids i do the same with baby food - freeze little portions for her bub too - she also makes some which i take (but i do a lot more) so i 'sold' the help to her that we can give them more variety that way
3. i enrolled in music classes with her and her baby - 1 activity a week we would do together that was bonding for her and baby too (we dont do this anymore as we are both back at work)
4. drop by for coffee or have her over for coffee and a chat - trying to avoid baby talk most of the time
5. I take my DS to the park near her house and drop past and take her baby too - so she gets even 30 mins to herself to make calls, shower etc etc - about once a week (i also sometimes do this on the weekend so she can spend time with her partner)

just by knowing you are there for her i am sure she is grateful and it is good she is getting professional help as well.