Here's what worked great on my paso fino:
For a few weeks, or even months, give her as much hay as she'd like and beep pulp instead of grain, soaked in hot water for 4 hours, drained, and fed with a cup of oats. I found one feed scoop full of beet pulp an appropiate amount, you may want to experiment a bit. Also, add into his diet Focus Senior or Focus Weight Gain supplements, which can be bought at the local farm or tack shop. When you feel she's at a healthy weight, slowly change her onto a diet of 2-3 flakes of hay twice a day, or however much hay you feel appropiate, and then you can go two ways for grain: The first is to use a good feed supplement like Grow 'n Win, if you feel she's not considered a senior yet, though at 11-14 years most horses become seniors. The second option, if you feel you should use a senior feed, is to use a good feed like Nutrena Senior feed. This has 14% protein and Omega 3 Fatty Acid, which will help the horse keep a consistant weight. Best of all about this feed is that your horse will NOT colic on this feed. I still feed my paso 2 whole feed scoops of it. Keep her on the Focus Senior/Weight Gain supplement as well. Crimped oats help put weight on a horse, feed a cup of oats with the grain each feeding. She'll come around
Hope this works, I know how it feels having an underweight horse and not being able to help him. My paso fino lost weight with too much activity trying to herd around a mare all day, and I went to countless equine seminars, taking notes on how 14% protein is better for a senior horse, as a lower protein level like 11, or even 8% is good for young, healthy horses....I use Nutrena Equine Senior feed on my senior horse and Grow 'n Win on my other 3 horses.