"She is my best friend."
Translation:C'est ma meilleure amie.
I am french and you can very well say : elle est ma meilleure amie.....c'est ma meilleure amie is a casual way of saying it when you are in front of the actual friend. Elle est ma meilleure amie could be said in a situation where you are talking about a friend who is not present.
Of course, the French say that. If you google "elle set ma meilleure amie" with double quotes, you will get 50k+ hits.
Hmmm...I can say the C'est-Ce sont/Il est-ils sont is one of the hardest (to me :0 ) to get through. My plan is spend full week to learn get to the...bottom (:0) of it until I can say I understand the whole rule. Wish me luck! (Thks for all the discussions)