I have an old gal whos sick right now- And I'm looking for ways to get her weight back up- Shes literal skin and bone right now and its hard to watch... I cant imagine its very comfortable either so I'm looking for ideas !
Sorry to hear about your girl I've had some success helping old/sick rats gain weight with a 'baby food bowl' (just a bowl filled with baby food), and feeding ensure with a syringe. I would want to make sure food is easily accessible for her and her cagemates aren't interfering with her eating. You may need to make sure she has time to eat away from any other ratties.
I usually start my compromised rats on lab block mush, with additives to make it tastier. For eg, once I make the mush,, I add flavoured baby cereal as well to make it more appetizing. This seems to work very well and I know my sickies are still getting a balanced diet in a form they can eat easily.