I'm going to give a criticism on a relationship
Is it your relationship or someone elses?
Yes, I want to tell him that he can do better than this foolish girl
You shouldn't be nosy and tell others who they can or cannot be with. It's not your call and if you do it, it makes you look controlling and arrogant. Plus, your friend probably doesn't appreciate you being nosy.
Whatever, I'm criticizing the girl because I can.
It is wrong to criticize someone else if you don't know the dynamics of their relationship. You may have things wrong and spread rumors.