Whether or not Walmart has the right to do what they did is irrelevant. It is still immoral because they are hurting this family. If Walmart's insurance was so good then this discussion wouldn't even be taking place, the whole thing would be a wash from the perspective of care.

Now however, the family won't be able to pay for certain aspects of her care (eg. private room). Therefore, Walmart is not simply asking for their money back, they are directly impacting the level of care this woman receives.

I don't understand how anyone could argue from that perspective that Walmart is acting morally.