Strip grazing! Tape her in a small homemade paddock :)
My old Shetland gelding was FAT and eventually we put him in a sand paddock (a big one) and a couple of hours a day on grass
Soaked hay and little to no grain
Put her on a dirt yard overnight with ad lib soaked hay (soaking it for at least an hour takes around 40% of the sugar out), and out with a grazing muzzle for about 6 hours during the day. Put her out in the morning if you can as the grass has a higher sugar content in the evening. Feed her a balancer pellet (zero cal supplement) to make sure she's getting what she needs. If you can, hand walking her even for ten or fifteen minutes a day will be great for her physically and emotionally!
Lock up of a day and let out of a night