I can understand why you are upset about finding out about the DNR the way you did.
I have had the DNR conversation three times now - once in January with my husband's consultant at the Community Mental Health Team, a few days later at the hospital he was admitted to in January - then a few days ago at the current hospital he is in. I had thought that once it was on his file once that would be it, but I was told that they are not transferable and can expire.
Thinking back to all three conversations, although I was involved and fully briefed as to why a DNR would be in my husband's best interests, I was never actually asked for my permission (not an issue in our case because I was fully behind the reasons given - having said that, I suspect I would have been overruled had I objected). Also, I was not asked to sign or witness anything as POA - it was the doctor alone that signed the form - again leading me to think that that final decision is with the most senior health professional.
Looking at one of the forms I currently have, while there is a section to summarise what communications were made with the patient or POA it also gives an opportunity to state why any such decision has not been discussed. So, it would seem there are situations when that might happen.
Forgive me, but I don't know your mother's exact situation - was she deemed to still have mental capacity at the time the DNR was issued? If so, the medical profession would have discussed things directly with her as the H&W POA doesn't come into play until all mental capacity is lost.
It's also worth knowing that DNRs are not automatically permanent. The one I have has a section where a review date can be entered although, in my husband's case, this has been given as 'indefinite '