Yes it's a difficult one, but you need to stay out of it, it's your daughter's choice, she is living with this man, and your job is to love the grandchild and love your daughter but stay out of the criticisms of her husband. I am sure she realises her mistake but she is being responsible , and you really need to just be loving and non judgemental over this situation. Her husband's mother probably realises her mistake too, but she is also holding onto her son, by paying for all these things. This is probably her way of staying involved in your lives.
Your daughter will feel more comfort and love for you if you don't condem him.
If the burden of carrying her husband becomes too much and she then asks you for help, then you have the right to offer help to her, in what ever form you want to.
Even if your daughter complains about him to you, just listen and don't suggest anything that might be used against you, because ultimately she will stick by her husband and you will be on the outer then.