This is an immigration question regarding testimony by affidavit and not a criminal law question. I recommend reposting under immigration.
This is not a DUI question. If you want an accurate response you need to post in a correct category. Witness affidavits in general, though, are sworn to be truthful. A properly completed affidavit would contain language to the effect of "I, Name, under penalty of perjury under the laws of the state STATE, swear the the information contained is true and correct to the best of my knowledge." Not only would that be bad for her, but it may come back and show negatively on you.
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This is not really an immigration question though framed as one. In any state and Federal jurisdiction, an affidavit is a sworn statement by a person who provides information to the best of his or her knowledge or ability including as to the source of such information. If the affidavit is not based on first hand knowledge, it is a hearsay. Hearsay is allowed in immigration context as long as the source of the information is clearly identified and a witness is available for cross examination, the affidavit is properly executed, provided as document for agency and not immigration court processing and/or the witness is classified as unavailable to testify.
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