That is not scale rot.
Here's some good ways to tell.
Is it all localized to the subcaudal scales?
Are there any signs of it on any of the upper body scales?
And sniff it.
Smell kinda rank?
Scale rot this advanced will be all over the snakes body and will not advance quite that quickly and suddenly over that span.
Have a hot rock or a UTH without a thermostat in there?
The snake needs stronger meds than that for this.
It needs baytril or amakacin given either injectible or orally for a few weeks until this clears up other wise it can turn nasty.
(oral and inj. is the same dosage, if oral meds, do not feed while on the medicine)
Have you changed the substrate to either news print or housing it in a temp home of a plastic tote with no substrate at all and heated with a heating pad on low or medium, depending on the thickness of the plastic?
Change the water daily, sterilizint the water bowl daily as well, using a bowl too small to soak in and go see the vet again at the FIRST sign of infection.
Which is probably already started, so you don't want it going systemaitic.