From a little experience I have, I would just like to add that in a serious case of depression, feelings, even love for family, or a pet, can be deadened. Everything, including packing and making arrangements to visit you, cooking, making decisions etc can seem overwhelming. Seeing other people in couples (like you and your husband), can make you feel even more lonely, and a divorced person can compare themselves with widows/widowers, even if their loss is as a result of tragic circumstances, by thinking that at least their partner did not choose to leave them, but yours did. That is a deep hurt that can smoulder for a long time beyond normal grief at being left alone.
I would suggest that your daughter may need professional help starting with her GP, who may refer her - she may need to be admitted to an appropriate institution for a short while, and they would hopefully attange counselling and follow up care, although what with cuts and shortages in the NHS, she may not get the help she needs or when she needs it. You and your daughters can support her as much as you can, but ultimately the patient will have to want to move forward. If she has a religion, is there anybody there that might help? Sometimes it is easier to talk and take advice from an outsider.