Given that the MIL had a DNR and that all there is left to do at this point is wait for her to pass, I don't think that it would be useful for the son to make such a petition.
Honestly, whether the husband presented the DNR or the medics brought it with them doesn't matter. The DNR existed because the wife wanted it to. Her requests were followed and it will soon lead to her death. It's sad, but it doesn't mean that the husband is the devil.
He might not be the devil but the FIL's 'story' just doesn't add up, he's not telling the truth. WHY? One has to carefully consider what he has to gain by lying about what happened. And how does the family even know that the supposed DNR was a valid instrument rendered by the patient? Is it in her handwriting? Does her signature look correct? Is it witnessed and/or notarized? The OP has some serious doubts about the situation as it was presented by the FIL, otherwise that first post would have been rendered very differently.
agnes!