How can you tell someone that there bad at singing?

There is this girl that Im friends with and well shes not particularly good at anything but believes that shes really good at singing. She doesn't have a bad voice but is nearly always off pitch and rather nasally. But that's just my opinion. I feel really bad telling her that's she good at singing because it just encourages her more and leads her to sing in front of the whole school who laugh at her which is very difficult to witness. I mean its not like her singing hurts anybody but shes gets insulted about it behind her back and I feel something needs to be done about it. Please help!