It is best to not let them start in the first place. This is where their freedom to make their own choices must be measured against consequences of actions which they lack the wisdom to make. Of course, having healthy role models is crucial for the smoker or would-be smoker. If the smoker is placed on oxygen, this presents an even more serious danger, not only to the person, but to everyone else around. Common sense says it's best to be niccotine free.
Sobriety means abstaining from alcohol at all times.
This is not advice just for the person with an alcohol problem. In fact, alcoholic drinking is not common among the PWS population. But light to moderate drinking can present problems. The nature of the syndrome leaves the person with poor judgment, warped perception of reality, poor balance, lack of impulse control, social immaturity, and emotional lability, and that's on a good day without any alcohol. Add the effects of drinking, and this could create situations that put the person with PWS at risk for serious problems.