In that case, you will not intervene to prevent that person from making that decision, because that person doesn't suffer from a permanent mental illness condition. Even though you say it doesn't happen to be remediable, in the context of somebody who is in the course of being terminally ill, that person's mental capacities totally change as much as the person who suffers from a mental condition.
On January 27th, 2016. See this statement in context.