Mom has a horse with heaves. Benadryl will work - a little. Tri Hist works in milder cases. Wheezy had used both, and albuterol in her feed, with limited results. This time of year is hardest on her, between the pollen, mold, and dust she struggles on a daily basis if I don't give her at least one dose every day of Dexamethasone. It's a prescription, and it's pretty cheap if you buy it in injectable form.(we pay around $15) One of the vets I know has been tinkering with it a lot, and has found that giving it to heavy horses in their feed, rather than in shot form, lasts longer, and works without the shock on their system.
It's also believed now, that heaves is an auto immune disorder. So, steroids like Dex is a reccomended course of treatment.
Talk to your vet, see if he/she can get some for you. Most horse vets keep a few bottles just for heavy horses.
And, of course, I have to say it, even though I'm sure you're already doing it - soak the hay she gets. Not just a quick wetting, Soak it. If all that doesn't work, she may need to move over to nearly all pellets. (what my vet said, so I don't want no flack from no one.) Flax seed supplements help too. The Omega 3 does wonders. There are lots of herbs that can be used... lots to try.
Ask Often, Demand Nothing, and Reward Generously.