The person would not be asking you to kill them, but to help them kill themselves.  I would think that you would want to refer the person to someone who would be able to deal with that patient more comfortably, rather than simply telling them that they must not do it.  Doctors who are willing to assist patients with suicide are not necessarily going to counsel suicide in every given case.   It's actually very possible that by sending that patient to a doctor who is willing to help, you might save that persons' life, because the other doctor might come across as not being judgemental and might actually do a better job than you in causing the person to reconsider.  
Referring a child to a cocaine dealer is very different than referring an adult to another doctor.   A cocaine dealer only sells cocaine, while a doctor provides counseling and advice as part of the service.  A doctor may change the mind of a patient regarding a course of action; a cocaine dealer will not.