Well if your hypothetical wife was traveling on a US passport, then there would be no need to get her DNI worked out. If she were planning on staying for a long time and wanted to get it worked out, then she could probably get it done through the Peruvian Consulate in the US. I believe it entails paying a fine for each missed election, but I may be mistaken, and I think they let it slide if you can prove that you were out of the country. But if she no longer has a valid Peruvian Passport (another thing that could be remedied at the Peruvian consulate but is probably not necessary if she travels on the US passport), I don't know because I believe that they give you the stickers that prove you voted in an election when you show them your passport saying you were out of the country.
Maybe, Send this hypothetical question to the Peruvian Consulate, they often respond to emails.