A couple of years ago, I had an elderly bunny with health issues who was losing weight, even though he ate his normal diet. I wanted to add something to his diet that might help him gain weight or not lose any more. After reading through Lucile Moore’s book on rabbit nutrition, and seeing the chapter about fats and proteins, where fat content of some foods is listed, I decided to start adding a few sunflower seed to his pellet bowl. Their fat content is one of the highest on her chart. I used plain/unsalted/raw sunflower seeds. I asked my vet about it and she agreed that it was worth a try, even though it’s usually recommended that rabbits don’t have nuts or seeds. That chart also lists almonds and pumpkin seeds as being high in fat, and are a little higher than sunflower seeds. Peanuts and walnuts are also very high in fat. I’m not sure if the sunflower seeds helped with the weight issue, because my bunny had a lot of things wrong with him, and at that point, I doubt that diet additives would have helped much.
I know that the jury is still out on whether or not the Oxbow Natural Science tablets help, but you might want to give them a try. Maybe the Multi-Vitamin or Senior Support would be the most helpful for helping with weight issues.
I’m sorry about the loss of your other bunny. It’s good that you have a strong bond with your bunny now, which will help both of you while you are grieving. I hope that she will start eating good soon.