Thankyou all for your ongoing input, it's really helpful.
This woman is not just a colleague - she's a friend in the sense that our families socialise often, I just wouldn't call her a close friend (we all have those friends, right? Who we know mainly thro our OH's, we like them but wouldn't necessarily be friends independently) and because she's senior to me at work that makes things more complicated/ awkward. However I do care about her, her DD and DH.
My sister had post natal depression so I'm quite in tune with the signs/ symptoms.
This woman has told me she feels really unsupported by her DH and her family (even tho they help with childcare) - that she feels alone and angry. She doesn't know how to look after her DD anymore. This has all come about in the last 6 months, around the same time it has become apparent that DD is missing milestones. Prior to that she was a completely different person.
Initially I'd say "don't worry, you're doing a great job", "we all have days like this", "let's go for a drink just us two" or "why don't I look after DD for a night so you and DH can have some quality time", but she's said "no, only I can put DD to bed and if she wakes at night she goes beserk if I'm not there".
I also gently suggested she think about speaking to a counsellor - I saw one after my Mothers death and it helped hugely - but she just brushed that off.
The DH also bought DD's progress up with me at a social occasion recently (whilst she wasn't there) - he said he was worried but also "didn't want to upset DW as she doesn't trust GP's and only wants to see the Pead Chiro"
Other people have also started to notice and gossip behind her back - people at work who think her work attitude has changed dramatically, and people we know socially who are surprised by the way she has been speaking to her DD & DH over the past few months. I can't stand gossip, so I've refused to get involved.
So yes, I'm worried that her drastic change in personality is a sign of something deeper (just as it was with my sister weeks after giving birth) and that she doesn't feel like she is supported by her DH, parents etc. I thought perhaps if she were to see a GP - or similar - they might be able to give her some practical advice on how to support her daughters unique needs and development, and maybe listen a little to how she is feeling about herself and her abilities as a Mother. But maybe that's naive of me.
I know how I might sound - interfering and/or naive. I probably am the latter but my intentions are pure. My DH says I shouldn't worry so much about other people. Perhaps he's right. Thankyou all again, this has been so helpful.