I have never liked hot rocks, and stopped using them about 14 years ago. The main problem is that a snakes ventral surface isn't very sensitive to hot surfaces, so they will sit on something that is slowly burning them and not know it.
Hot rocks will also quickly dry out the shedding oils if a snake coils on it for long periods. This causes the new skin to be pulled off with the old, thus causing nasty damaged to the ventral scales, leading to severe scarring.
As nasty as the photo looks, it will heal okay with successive sheds. All the scabs will come away with the next shed, leaving raw flesh exposed. But this will dry, and get better with the following shed. In time it will look much better, but scars will remain.