Should I tell my friend that her friend is stupid ?
Why would you tell her that her friend is stupid?
I think she is the dumbest person on the planet and my friend does not know it.
You should not call another person stupid. It is rude and disrespectful to do so.
Come on, how is that going to hurt her? It's fine for me to tell my friend about how much of a dumbass her friend is.
She's your friend's friend so let your friend decide if that person is good or not. If you don't want to hurt your friend's feeling you won't call her friend names.