A good friend of mine (and neighbor) had terrible PPD and it was so difficult on the whole family. She got through it and we were having a glass of wine one night and talking pretty openly about it.
Some of the things that helped her:
1. Meds and regular visits with her doctor to check-in
2. People who remembered to ask how SHE was, not just the baby
3. People who talked to her about things going on in the world (or in our lives), not just "baby talk"
4. Visits from friends
5. Offers to babysit so she could get out for a while (may be tough for you to do this for her, but you could offer to take the 2 yr old again)
6. Offers for other husbands to take her husband out of the house for a while b/c he needed to "talk" too and she just could be the "listener" at the time.
Focus on helping her get through this, especially if she has help at home. She has shared the PPD with you, so ask her about it. Check in on her regularly and come to her with specific offers. "Sara, I made a double batch of lasagna last night and thought I would drop it by today. Does 4:30 work for you?" People suffering from depression won't "initiate" interactions, but will welcome them if you initiate.