Substance use is a broad spectrum, and at the soft end (for example half a bottle of wine when you get in from work every day) it's more straightforward to look at it as a behaviour and think about other options (such as go for a walk). If the person's using alcohol to manage hearing voices, psychosis or intrusive thoughts, it's much more complex and from a purely humane viewpoint I really think they need support with their mental health issues to be able to start managing the substance use - not forgetting that a very heavy drinker must cut down gradually as stopping suddenly has its own risks, and that needs monitoring by a doctor or nurse.