Is she eating and drinking? Can you feel of her crop in the early morning to see if her crop empties overnight? Let us know what you find. If it is still firm in the morning before she eats or drinks, you can give her some chunks of chilled coconut oil, or some mineral or olive oil in some cooked egg. Then massage the crop several times a day. If it is empty, then that is normal. The presence of a tumor may not necessarily end her life in a short time. If she has a crop disorder, that could cause weight loss.
There are a number of ways that people use to humanely end a chicken’s life. Many remove the head with garden loppers. Some use the broomstick method or cervical dislocation. I hang them upside down by the feet from a nail on a tree, which seems to calm them, and cut the jugular veins with a sharp knife. They bleed out in a minute or so. You can find more info about those methods with a search online.