First, for anything in "nature" to evolve it has to make babies. Lots of them. Over many generations (not wanting to get into a discussion about bottle necks here). So this means we need our snake hair woman to not be one of a kind, but part of a snake hair people population. Though commonly displayed as a "woman" who's to say they don't reproduce asexually? 

If they do have sex, that's another good indication that they have some sort of social structure.  Further assuming that they are at least in the ball park of intellectual capacity of mammals, social evolution is something that happens at a much faster time scale than biological evolution. 

I like the idea of a symbiotic relationship, but more so in the less "natural" way and more in an agricultural sense.  Perhaps the snake people have evolved with snakes in the same way humans have evolved with dogs (thousands of years of domestication changed us as well as them). 

I further like the idea of a teen snake person getting their first snakes to roost on their head. A certain amount of training, education and physical tolerance might go into this ("Sally, did you swallow your daily viper venom?").

As for what the snakes get out of it, I would imagine the heat generated from a warm blooded mammal like animal, food, protection from predators and access to mates.  Pretty much the same things most domesticated animals get.

For turning to stone, I recommend a toxin that induces calcification. Perhaps the snakes are trained to spit in the eyes of animals that face them (also not wanting to get into a debate over what constitutes spitting). 

Further snakes could be trained to be close to silent (many snakes already are), and convey hunting information and predation warning to their handlers.