I agree, free choice quality grass or timothy hay with beet pulp and I would add alfalfa pellets. They will provide extra nutrients and calories, plus make up for any lack of nutrients in your hay.
A good, affordable vitamin supplement is Balance II from horse.com. It's broad spectrum and cheap! I used it for a while and it seemed to work well.
I would start her out by giving her half of her normal sweet feed ration mixed with 1 lb of shredded beet pulp and 1/2 lb of alfalfa pellets. You can feed beet pulp dry or soak it for 15-20 minutes in water. Pour off the excess and add the rest of her feed. Keep her on this for 1 week, then stop the sweet feed completely.
After the first week, increase the beet pulp to 3 lbs and the alfalfa pellets to 2 lbs. That with your vitamin supplement should bring her weight up nicely without the excess energy. Once she's up to weight, cut out the beet pulp completely and stick with the 2 lbs of alfalfa pellets. Add a little water to get the vitamins to stick.