When people are sick it happens for a reason, that's like playing God. It's not up to them to decide when to live or die. They didn't decide when they were born so it's not right to decide when to die. God has a plan for everyone and assisted suicide would be interfering with that plan.
Suicide is bad.
You only have one chance at life. If they are sick they can find medicine or get counseling. If they are religious they should go back to their holy book to see what to do. If someone wants to die that bad, they're going to die. Going through the process of assisted suicide may help them decide if they really want to live or die and bring them closer to a more secure decision.