How is she doing?
With her age, not laying eggs in a while, the weight loss and the color of that poop, I would lean toward reproductive problems, but it's really hard to know. Can you feel her abdomen between her legs, below the vent - any swelling or feeling of fluid there?
I would try to...
Welcome To BYC, I'm sorry to hear about your hen :hugs
Where are you located in the world, that is helpful when making recommendations.
If you can get a photo of her poop, that may be helpful too.
Check her crop and tell us what that feels like - is it empty, hard, squishy, doughy, fluid...