I'd be concerned enough to isolate my horse from the one experiencing difficulties. At the very worst, it could be foot and mouth at the other end of the seriousness spectrum, it could be the horse has been in white clover or other plant that pis home to fungal infections which the horse has ingested (easily treatable).
Additionally, I would hope the owner seeks help from a veterinarian as soon as possible.
The affliction also sounds like a possible case of vesicular stomatitis (VS), a highly contagious disease. Although I doubt the black colored tongue is indicative, but rather secondary (if at all), a thorough examination of the horses mouth needs to be accomplished and this, especially if the horses mouth is really sore, can only be done under sedation.
Here in the western U.S., we have ranch working horses. In the spring another type of stomatitis can be encountered if the horse is in a pasture that has broomtail, foxtail or other grasses that have awns... the sharp ended seed pods that flare and give the grasses the distinctive "tail" look. These can become imbedded in the lips, mouth and tounge of the horse. A typical diagnostic observation is the horse tends to slobber a great deal. Making sure the horse has a lot of water is important, but, again, an examination by a vet is paramount...
Good luck!