How would you tell someone that he sings awful without hurting his feelings?

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I have a really good friend, and He thought he got excellent voice but to be honest It was the exact opposite. I once joked about it, to tell him that he's not that good, because i don't what other people tease him or make fun of him. But he didn't took it very well and i felt bad.We're OK now but He still sings bad, and he still likes to sing too much especially on Friday nights outs.