If it is fungal, the question is what is the underlying cause? They should be fighting off that sort of infection themselves, so has something weakened her immune system or is there another problem which allowed the fungi to get a hold?
Obviously if it's not a fungal infection the question is still, what is it?

I lost a rainbow fish yesterday to a fungal infection, all the others look fine, so I'm assuming she had some condition which allowed the fungi to take hold on her quickly. Sounds bad, but all I thought with her was "I'm glad it's not one of the snakes".

I hope you track down the cause soon. If you do decide to treat the area with an antifungal a lot of them are antibacterial too, I'm wondering if you could use some of the aquatic treatments on a snake's skin (off the top of my head, gentian violet has antifungal and antibacterial properties).