Mouthrot is treated specifically by a veterinarian. You probably can't choose over the counter meds by yourself without a vets consultation. Like scalerot, it is due to too damp or nasty conditions in the tank. I don't suggest purchasing a sick animal, but you can suggest them to put that snake on papertowels or newspaper and try to get it to a vet. It should not be in a cage with other snakes anyway, so if it is, suggest that change as well.