Hello. Thanks for the question.
That is a significant amount of weight loss in a short period of time. That is not going to be diet related. That's almost 15% of her body weight. To have that degree of weight loss, I would worry about some type of cancer or metabolic disease. I would first do full bloodwork to check organ function and to look for markers that may point to thyroid or adrenal disease.
If the bloodwork is normal, then cancer moves up to the top of the differential list. The hard part is finding it. I usually recommend chest x-rays and an abdominal ultrasound. This will evaluate both body cavities to look for masses. Anything found can be aspirated or biopsied with a needle to find out what it is.
If all this has been done, then I would consider an abdominal exploratory surgery. This is to get biopsy samples of the intestines, liver, lymph node and stomach. If there is a disease in the abdomen, it will be found. It's an invasive diagnostic test, but sometimes this is the last step to get an answer. If you don't want to go that route, then starting some steroids (Prednisone) may be a palliative therapy option. We don't know what we're treating, but we'd be thinking some type of auto-immune or cancerous process and we're treating the treatable things.
If you have copies of bloodwork and x-rays to post here, I'd be happy to lend a second opinion on them.
I hope this helps, let me know if you have any other questions.