"Su mente se compone" refers that or I assume you can say "la decisión está tomada para ella".

"Her mind is made up" is an expression which can't be translated to spanish literally, therefore you should see the context into that phrase. You want to say that a girl has a decision in mind, therefore, that sentence could be translated like this: "Ella tomó una decisión" (when you don't know about the decision she took) "Ella tomó la decisión" (When you already know the decision she took) I hope I'd helped you!

Se ha decidido.