The short answer would be that you are probably correct in your assesment and unfortunately there is really not much you can do about it if she doesn't want help.
As for what to do, that is a tough one. Does your company have any kind of drug policy? If so you could require her to take a drug test and if your assessment is correct require her to seek some kind of treatment as a condition of her continued employment. Will this work? Hard to say. As I said before, you really can't force someone to get clean if they don't want it, but you can "force her hand" so to speak.
If nothing else you have the interests of your company to look out for. Hopefully you can convince her to seek help, but it is far from a guarantee. Others here may have other ideas, perhaps better ideas, for you. Take care.