Help! This is a matter of life or Death!!!!!!!!!!!!!?

So, my little sister,KK, and her imaginary friend got in a fight over a boy in her cl. So my sister ended up getting the boy, while the imaginary friend, Donna, ended up getting mad at my sister. So this morning KK tells me Donna is going to hang herself...b/c she hates her life. I should mention that my little sister is 13...yes She still has an imaginary friend...yeah I know its weird. So What should I tell KK to do about this matter of life and death????? Please help, if Donna kills herself my little sis will be devastated.