No other conditions. Maybe a 4 week waiting period to not have people killing themselves due to temporary unhappiness. Otherwise, no conditions, no age requirement, or 'sanity' requirement.
I can think of nothing more inhumane than forcing someone to live against their wishes.
In the end, Rámon Sampredo (the protagonist), who's been paralysed from the neck down for close to thirty years, commits suicide by drinking a poisonous liquid through a straw from a glass standing on a table right next to his bed. Friends of his prepared the liquid and placed the glass right next to him, but ultimately the decision to drinking the liquid is his. I'd have a hard time convicting his friends for anything, other than being good friends. "The basis of optimism is sheer terror" - Oscar Wilde
