When a snake goes off feed and has minimal to no weight loss, that's pretty normal for a winter/breeding fast. The huge change in weight could mean incorrect husbandry and/or parasites.

Please do not euthanize her. Parasites are a very minor issue. It's one of the easiest things to treat when it comes to veterinary medicine. If you have any questions regarding parasites, email any big named breeder. They import WC girls all the time and treat them for parasites before selling them. (Outback Reptiles comes to mind)

How much weight has she lost total? What was her starting weight and what is it now?
Can you describe your housing, husbandry, temps, etc etc?