Quitting is not an option for one to keep moving forward.

Pending on the substance, I think that family doctors are quite well versed in helping at least alleviating symptoms of withdrawal - coming off a drug - and there are prescriptions available in order to soften the blow so that there's not a complete drop into nothingness.

I'm not sure if my thoughts help, but it's merely to say, I would hope that somewhere there could be a happy medium where your friend's brain is not "fried" and nor is your friend flung into utter anxiety with physical symptoms.