Should I let my friend do this?

I always talk him out of suicide. He always say his life gets worse and worse. I told him, the first time he tried to kill himself, that I would follow after. So he told me that we would die together if we finally had enough. So, now, my friend is depressed because his little sister died. He is begging me to let his life end. I keep begging him not to but he just told me " If I can't cheer up in a few days, I'm ending my life. If you really loved me, you would let me die." I don't know what to say! Please help me.