I sympathize with your situation, I wish there was more that could be done. I will be completely honest, if your friend was caught being under the influence of drugs she would likely be arrested, booked, charged and released after a day or so. At that point a public defender would strike a deal, and due to overcrowding it's unlikely that she'd serve any time at all. If this is your friend's first offense the chances are she would be asked to attend some self help meetings and a random drug test or two.
As for being taken into mental health custody, if she is an adult the chances of her actually being committed are slim to none. The system is broken, I know I'm a part of it, but it is not helpful to people in her situation. If you truly want to help your friend then advise her to seek some help, don't let the roommate get away with turning a blind eye, and do what you can to encourage her to get the help she needs. Believe me, I know that sounds unhelpful and impossible.