Mouthrot, never fun to deal with in reptiles. I would truly recommend finding yourself a reptile vet since the treatment isn't exactly easy.
But you also need to eliminate the cause of the mouthrot. There are a number of causes, so you need to narrow down which it is. Improper nutrition, incorrect temperatures, overcrowding, trauma, or parasites can all cause it. So you need to evaluate the snake's diet to make sure it is balanced (proper calcium/phosphorus ratio is often an issue). Are you maintaining a proper environment? Temperature gradients, day/light cycles, and humidity? Is there adequate cover so the snake doesn't feel exposed and stressed? A vet can run a fecal for internal parasites, while you can mechanically check for external parasites.
Treatment involves debridement of the lesions (removing dead/damaged tissue so healthy can grow in), daily antiseptic flushes (using chlorhexidine or other mouth safe antiseptics), and antibiotics (topical or injection). If left untreated, the animal will die since the infection will spread from soft tissues like the gums to the bone underneath. Not a pleasant way to die, fyi.
