I'm going to tell my daughter a guy who rejected her didn't deserve her
Is your daughter okay? Why do you need to tell her that he doesn't deserve her?
He must be a dummy to reject someone as lovely as my daughter is.
I do not know who rejected her, but you may say that to make her feel better about the breakup. I hope you support her and help her cope if she would have other breakups in the future.
I'm going to tell her that he sucks. He will not have someone else better like my daughter.
Then you can say whatever you want to make your daughter feel better, and maybe guide her to have better choices in the future so that she would experience less heartbreak in her life.