In my opinion, you're correct. She is only eating one particular sandwich, and it's identified in your first sentence. The definite article is correct.
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AvangiTo say that the sandwich which is affected by the action is the same one Mary is eating is simply a better, more complete answer.But the asker is, supposedly, unaware of that.. .
Avangithen I'd be inclined to direct the student toward a sandwich.Hi Avangi,