Well if he loses his leg and doesn't want to live because of that, then he needs some counselling and proper care to lift him out of depression rather than anything else.

As it's against the law to take a life except in the most extreame circumstances where no hope of recovery is possible and it's needed to end constant suffering, he can't get 'his wish to die'.

This also answers the second scenario, except that any family the braindead person has will be the ones to consult about pulling the plug.