I feel for your situation! I hope she gets better (I'm sure she will under the vet's care).
We just had a similar situation with a calf that was abandoned here about two weeks ago. I think someone just dropped her off at our farm who didn't want her (we called all the livestock auctions and everyone we could think of who might have owned her - nada). Anyways, the calf was a few days old and had bad diarrhea.
We got advice and basically bought calf replacer (milk-based, NOT soy-based) and gave that 3 times a day, plus gave her some pepto bismo and 2 antibiotic shots which we happened to get free from a vet because it had just expired. We also added a raw egg to her warmed milk which they advised. She was pretty sick for a few days, but gradually she recovered and now she's doing great and we found a home for her with a lady who has a couple of pet horses and sheep.
By the way, I'd just like to add my 2 cents about milk powder versus fresh raw milk discussion. The idea that "real" milk isn't as good as a powder is a sign to me that we have been truly brainwashed by a lot of hype and marketing. Real milk (and milk with colostrum) will always be superior to powdered formula. If you're looking for medications, and you want powdered milk with medication added, well I can understand why you might want to buy premixed powdered milk. Dairy farmers feed their calves milk replacer because the real deal is what they are selling to make a living.