OMG You've got a 3 year old with a history of repeat founder on COB...and your vet hasn't discussed diet with you? I find that hard to believe.
If she is foundering again, you don't need to wait until Wednesday to have the vet out. You need to have them out today for immediate treatment.
You need to remove all grains from the diet just to be on the safe side. Insulin Resistance might be a factor and if so then feeding COB or grains and allowing free-choice pasture are the LAST things you want to do because you send the risk of repeat founder right through the roof. If you can get your hands on Bermuda grass hay, that would be a better choice for forage though it could still have too many non-structural carbs. If you don't have bermuda grass hay, go ahead and soak your hay for 30 minutes prior to feed. And supplement with a simple ration balancer designed to go along with grass hays. Feeding COB is actually likely to be making even the "weight" issue worse because you are throwing the metabolism into disarray and causing muscle wasting.
Please read this article: www.thehorse.com/pdf/aaep/obese.pf
Note, not all horses with insulin resistance are fat.....in uncontrolled disease there can be significant muscle wasting leading to a very thin appearing horse.