Okay, my friends and I are TRYING TO start a band and I'm having a problem...?

One of my friends, let's call her Mandy (that's not her name, just a name I'm using). Mandy wants to be a lead singer, but... she sounds horrible. I don't know what to do! I love this friend as my sister! I don't wanna hurt her feelings, but I don't want her to get embarred either. I know I should not just worry and let her do what she wants but I'm just really stressed lately. Ugh, any advice?