RE: attachment theory, its widely accepted that children choose a 'primary' attachment figure from around 3-7 months, but are able to form more than one attachment past this stage. So that can include another parent, grandparents or nursery staff. Putting children into nursery has not shown to have any detrimental affect on attachment to the primary figure, usually the mother. Attachment to the mother depends on her responsiveness to the babies needs. So a child being in daycare will not cause attachment issues.
As for it being a long day, that seems to be depend on people's personal opinions. At DDs nursery she will be following the same routine as at home for naps and meal times and in between will be playing, having story time and playing in the garden, just like she would at home, it's not like school so I personally don't see how it can really be a 'long day'. There is also the added benefit of other children around, which she doesn't have at home.
Some of the comments on here are so judgemental, particularly the remark of how the OPs DIL is 'doing a bad job'. As I'm sure many of you know, being a mother isn't always easy, being a working mother is harder. It's horrid to see people making comments about why did she bother to have children. I'm sure she loves her child, just like anyone else who goes to work and has to put their child in nursery, and I'm sure she has her reasons for putting him there in the holidays. But people aren't expecting her partner to be using his holiday to be there for the child are they?
The OP has said that they worry her DIL has PND, what if she does? What if she is continuing the nursery place to give her the head space to be a good mum when he's home? At least his in a place where he is well cared for. There are plenty of children in far worse situation than this, so the saying 'poor child', 'why have children', she's 'doing a bad job' is really quite uncalled for.