Slow feeder first and foremost, more regular exercise to keep her moving like an hour every day, and up her protein. I would use a bit of Alfalfa hay mixed in her slow feeder net to be sure she is getting adequate protein. Her overall body condition would determin how much I would give her. Poor topline but fat belly would make me give more than a fat horse with a big belly.
Also, deworm well with something like Quest Plus that gets all worms or do a power pack (quest works just fine and is cheaper) and make sure the horse is clean.
Feed a daily probiotic toaid digestion.
Also, plain grass hay is usually low on certain nutrients. Id be inclined to use a ration balancer daily to make sure she is getting adequate nutrition. It is possible for a horse to have tons to eat and still be malnourished if the forage doesnt provide enough nutrition. Usually, hay belly is caused by low protien content and the horse overeating to try to get what it needs.
A good evaluation of the horses nutrition is prbably a good idea but a slow feeder, a daily quality ration balancer and some alfalfa usually does the trick.
Last thing is again the quality of the hay she is eating. If it is mostly fibrous over mature stems and barely any leaf blades, she wont be able to help but to have a hay belly because the forage is very indigestable and mostly bulky fiber. The way to tell is if you dig your fingers into the hay and it feels prickly, it is not good horse quality hay and is overmature or weedy hay. Good hay should be soft when you dig your fingers into it.
Long story short, Hay belly is 99% a management issue. You just have to figure out what the cause is and methodically change things till you get it.