Additionally, it is present in hay and grain due to the way these products are harvested. So, killing opposums is not going to protect your horse. The main way to protect your horses is to keep them healthy, good weight, and don't excessively stress them out.
As far as treatment goes, the recommended treatment is Marquis with 10,000 IU plus of vitamin E. Marquis should be given for two months straight. This will costs approximately $1200 if you buy the Maquis online. You can get a prescription from your vet. However, I will add that in order for a horse to be properly diagnosed, it MUST be spinal tested. Blood tests are not effective as almost ALL horses will test positive for blood exposure. EPM sets in when the organism breaks the blood/spinal barrier.
Your friend can e-mail me privately to discuss the disease further if she would like. I am, unfortunately, very experienced in this area.