dannyK @0743 makes a good point in my opinion as there is depression and there is depression.
If she is already suffering from a depressive state at the moment and that is ,say bipolar, then she is much more likely to suffer from Post Natal Depression, although any woman post partum can suffer from depression.
My advice would be to talk it over with a professional.
I was 34 when I had my daughter and 37 when my son arrived. I was regarded as an older mother but everything was fine with them both. The only problem I see is that I am 70 now with two young grandsons I may not see grow up.
Had my child when I was 42. I had a life first! Its been great. She has kept me young. Even when Ive had to drive around the clubs in Ibiza at 5am. Go for it!!
My wife was 35 on number 1 and 39 on 2 - someone I know in the USA was 52 when she had her sproglet in the middle of Hurricane Ike and her hubbie and her lived on two boats south-east of Houston.