Son in law issues

I would tell him that you are not able to help him and refrain from these conversations moving forward

The element of perceived betrayal is very dangerous here, given the circumstances they live now

As for your help causing resentment - from him or your daughter too?

Walk away and refuse to engage with him unless your daughter is in the room with you both.
If he runs her down whilst she is there too, then you can tell him in front of her that he is out of order.
Is he phoning you? Email? Or does he come round on his own ?
If he is making a special trip to run your daughter down, tell him to go home and as I said above don’t engage.
And don’t get in the middle of it, let them sort it out between themselves.

You listen to him as a sounding board, as devil's advocate, to a man whose wife has just gone through a difficult pregnancy and birth, and who clearly can't cope with being a father?

Don't listen, don't comment, don't get involved; tell him it is hard for both of them and simply offer to do something such as shopping, washing etc, so they can both be involved with the baby.
Is there an adult male, brother, father in law who can tell him in a few trenchant phrases to grow up and accept responsibility as the father?

I agree with Oopsadaisy don't engage with him unless your daughter is there with you. Let them sort things out themselves. It's hard to stand back I know Sharina.

I think you're being very unwise listening to him, I would not engage, be tempted to just tell him to grow up and man up, that could rebound on you and your d though, better still as him it to himself as your priority us your d and gd you are worried about them. He sounds very immature and spiteful, it's all about him.

Message withdrawn at poster's request.

I would say gently but firmly that you are sorry but you won't get involved between them, its not fair to you to grumble about your daughter behind her back, kindly acknowledge they are both going through a tough time, and suggest they seek some marriage help/support. They need to take their difficulties together to someone.