Sounds more like she started out with belly rot and it's gotten to this horrible stage....likely dirty wet bedding is the culprit here or a burn from a heat rock started this whole mess. Poor thing!

If your vet is advising that there is not much more than can be done for her and since the vet's actually had his/her hands on this snake, I would follow that advice and have her humanely euthanized as soon as possible.

Sadly we can't save them all and we, as the responsible parties, must know when enough is enough. Please know that we all feel for you having to make this difficult choice after having struggled to save this snake.