She was making a dinner before I arrived or She made a dinner before I arrived.
No need to use 'a'.
She was making dinner before I arrived: She started making dinner before you arrived. The dinner is probably not yet ready.
She made dinner before I arrived: The dinner was already ready when you arrived.
If the dinner was still being prepared: "She was making dinner when I arrived."
If either the preparation was complete, or maybe the dinner was in an oven:
"She had made dinner before I arrived." or "Dinner was [almost] ready when I arrived."
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