How can I tell my friend that he cannot sing, at least without professional help?

Okay. My friend and I are starting our own music group and we already have a great singer. I mean the guy we have has "pipes of gold". But here is the problem: my friend reallly wants to be the lead singer. However, I have tried so hard to be open minded, but without trying to sound like a jerk, my friend truly does have a horrible singing voice. My friend seems to be delusional and thinks that he has a great voice and does not pick up any of my hints that he should pursue other fields. It seems like everytime we hang out, he probes me to tell him that he has great singing voice. I don't want to put him down, but I also don't want to be dishonest. I have suggested that he consider getting professional training, but he insists tha he does not need it. What should I do? I can tell that his family and friends and feel the same way as I do, but do I have to tell him the truth? What should I do?