HI! I'm sorry about your horse. Foaming at the mouth and nose means the horse may have been poisoned. Since a horse can not vomit, the toxic poison would come back up the throat and out the nose. I don't know if this is the exact cause, but I would put monies on it. Since it happened so fast, I believe this is the most likely cause. Without a toxicology report, you have no way to know for sure. But it sounds the most reasonable. Please consider the outcome before you accuse anyone. Without proof, you can not point fingers. Consider a tox screen. If you have any other questions, please ask. I will help in any way I can.
Please remember to click the ACCEPT if this helped you and I get credit for it. Thanks.