I think what I suffered was actually PTSD (that occured when DD was a few months old) coupled with massive sleep deprivation. For me, what I really wanted was someone to look after DD while I slept and/or had some "me" time. I think that's probably the biggest thing you could do for her. Or perhaps her partner (does she have one?) can take her baby, you can find someone to watch yours and the two of you can go out to lunch or a movie? Feeling connected to the world again would help, i'd think, plus the adult friendship and company without constantly worrying about the next interruption from bubs!

She's very lucky to have such a caring friend as you.