My friend is a horrible singer but thinks she's good at it?

A "friend" of mine, who is completely immature and I can only deal with her in a happy, generous mood, thinks she is the sh*t at singing. However; she is the WORST singer I've ever heard, even with singing lessons. No-one EVER tells her how bad she is, but it's all they'll talk about. The thing is, my family does NOT get on well with her family, and she is so sensitive that if someone insults her favorite grass blade she'll burst into tears and scream about how horrible you are. She does not associate really with anyone over the age of 5, and no-one knows how to deal with her. But you have to feel kinda bad for her pathetic-ness I suppose, and I really want to spare her the pain of still trying out for our local "Idol" competition and singing in the streets (That hurts EVERYONE) and constantly being rejected from competitions (Yes. Even after all of that, she still thinks she's wonderful). Is there ANY way that I can get her to know how BAD of a singer she is?! I don't know HOW her parents let her try out for things.... It's gotten so bad I'm on the verge of paying someone to come up to her and tell her how bad she is. That may sound harsh, but she's living in a deluded fantasy land and she needs to GROW UP because when she hits the real world, she's going to be in for a shock. I'm hoping that knowing she SUCKS at it, will motivate her to try other things, and maybe develop a bit of a personality and maybe make a friend who ISN'T four? Even if it doesn't, I'll just be happy if she stops because she's just hurting herself and any unfourtunate persons ears.. And I'm not sure that if ONE person telling her will be enough, as she'll fall apart into pieces and to get her to stop sobbing hysterically sitting in the street waiting for a car to come and "end her horrible life" while hopelessly waiting for people to notice the "designer" jeans that are 4 sizes too small in an effort to make her look "skinnier" even though that is NOT how it works while dropping HUGE hints for you to notice them until all you can do is tell her not to listen to the "haters" so she'll just SHUT UP and go away. What to do?