Being a medical professional myself, I would remove the tail if I were you. While the tail is decaying, this doesn't mean it has NO blood flow. A significant decrease in blood flow can kill the tail leaving the ray in pain and discomfort. This doesn't mean sepsis cannot occur. The issue is that you have a raging infection next to an open wound. The bacteria and fungi don't have to be introduced, they are already there! By removing the tail, you are doing what's called debridement, which is removing necrotic (dead) tissue to improve healing. You're also removing the infection. The tail, even with its decrease of blood flow, is most likely causing the ray pain and discomfort at the attachment point and possible part of the tail. By removing it, it'll cause the ray pain initially but reducing the length of suffering.
You're right removing the tail can stress the ray out and kill it but I feel that leaving the tail on is more likely to lead to death. The ray is eating after having its tail mangled so I think it should recover fine.