If you are saying someone who is suffering from mental illness that look he has Cancer, what you have is normal, it happens with everyone, So Chill !!!
Don't do that... Really don't!!!
You don't even know what your friend, relative or colleague is suffering from. And you are not even helping, you are just judging and making things more complicated for him.
Try to understand other person. I know you have your own mess to sort out. But we can just listen to others without judging.