I think the guardian's wishes should be considered first, but in the case of something that is extremely terminal, they should not be in the care of a hospital but instead of a hospice, and I think a hospital should have the right to say, "I'm sorry but there is nothing we can do except for keep them alive just a little longer, but these resources are needed elsewhere for people with more promising outcomes. The condition is terminal and perhaps a hospice or staying at home with the family for their final days may be better."
Keep in mind that not all hospitals will choose to refuse to treat, even if they have a right to.