It's perfectly reasonable to do the fecal test to rule parasites out. There are so many possibilities, that we can't possibly cover all the contingencies. If after doing everything that you are comfortable doing, when supportive care is not getting any improvement, when you think quality of life is poor and/or she is suffering, then it's completely acceptable to end her suffering. We all have different levels of what we can, and are willing to do, when caring for a sick bird. I've nursed some for months for injuries, there are other keepers that might not do that. I've nursed some that did not make it ultimately. I've lost several to cancers and salpingitis. I've ended others quickly when I felt that there was nothing I could reasonable do to help them feel better and they were suffering. When something is going on internally it's often very difficult or impossible to know for sure what's going on until necropsy. When a bird stops eating or drinking, isolates themselves from the flock, is obviously unwell, then that's when I usually euthanize. I've had some birds with cancer or salpingitis live for 18 months before the time came. The fact that she's so thin suggests that it's been going on for awhile already.