I've had better luck with flax meal than rice bran. I'd add to his diet a good digestive aid, pro or prebiotic type supplement.
Are you giving her any grain or other feed? If not, I would use an all-around vit/min supplement like GrandVite or High Point
. High Point already has probiotics and yeast in it for digestion, so that would be a good all-in-one supplement. The NutraFlax product from Horsetech.com is also very good. I have liked everything I have gotten from them.
If you feed flax meal, you shouldn't need to also feed cocosoya. You can feed up to 8 oz of flax per day, which is about 2 cups. That's plenty of fat and calories.
If you're feeding grass hay, you might try some alfalfa pellets instead of, or along with, the beet pulp. I have had great luck putting weight on horses using a combination of alfalfa pellets and flax meal.