here's my advice:
What would be your advice to the girl "A"?
okay, first off, if the guy does not deserve you. you were loyal to him. same goes for your friend. she doesnt deserve you. she betrayed you. i suggest you try forgetting the boy.
Remember it's easier said than done. Please give her specific advice as to what she should do to overcome her depression.
First off, i think you should try and get closure. look around your room and other places you go, and try and remove all of the things that remind you off his betrayal. then, if you other friends, you should try to move on, hang out with ur friends more. focus on schoolwork. hang out more with your family. they will probably help you move on. take up some activty like swimming or painting. all of this will help you feel better.
Would forgiving them help her? As long as you don't forgive, there's this burden on your heart. And you don't have to forget while forgiving, right?
i think forgiving would be very hard at first. but eventually, you should forgive them both. be the bigger person, let your exboyfriend and friend know that you are strong. move on let this be a learning experience. next time, dont devote yourself to your boyfriend before you are absolutely sure he will stay with you, through thick and thin.