I'm going to tell my friend's parents that she has just given birth to an unwanted baby
Why do you want to tell her parents?
It's just what I think is right
Even if you think it is the right thing to do, it is not good to tattle on your friends to their parents. It would betray your friends trust and you could lose their friendship.
I don't mind her getting stressed by her parents, it's payback for the shit she did to me.
It's wrong to seek revenge on someone. When they find out what you did they will try to get you back and it doesn't end.