N565 case!!!!! Visiting Mexico


my parent has an open case for replacing his naturalization/citizenship document. He didn't attend his interview therefore he doesn't have a replacement. He can't get his visa because of that but he is traveling to Mexico. What can he do in the meantime to show he is a citizen of this country?
No, my dad's paper from years ago that stated he was a citizen had faded words so when he requested a passport he couldn't get one because they said it was part illegible. So he filled out an N565 form which is to get a replacement document of his original. His case is still open with the USCIS and has not received his replacement.

In the meantime, that original document was what he used whenever he would travel to Mexico and back instead of a passport but now he doesn't have the original document because when he filled out the N565 form, they required he turn in the original document. Now my dad only has a receipt of him turning in that N565 form and it's stamped and everything, but it's just that, a receipt.

My question is, since my dad doesn't have his original document and only has the receipt of his N565 form, what can he use to show his citizenship when he returns from Mexico? He is currently traveling there.

Sorry for the long explanation, hopefully it isn't confusing!
No he doesn't because his case to get a new copy of his cert of nat is still under review. He only has the receipt/document that states he went to his meeting/interview to get a new copy and that's it. The original copy is with the USCIS.