For long-term treatment (because if you quit treating when it looks like it's gone, it WILL come back) you can use Hibiclens. You can buy it at most pharmacies. It's milder than other treatments, so it's better suited for long-term treatment IMO. It's good to start with the treatment Albert recommended and then pick up the Hibiclens after the major symptoms have been treated.

Additionally, I'm sure you already know about this, but you need to use separate equipment for this snake. If it's truly mouth rot and not just a mouth infection it is highly contagious.