What does it say exactly?
I just pulled up a copy of mine and it does say:
"Evidence of your sponsor's and/or co-sponsor's United States Citizenship or Lawful Permanent Resident status.", the thing is that is a bullet point associated with an I-864, since this is a VAWA and you've filed an I-864W this is not applicable. If you haven't filed an I-864W, print one out, fill it out, sign, and bring it to the interview with you.
If it does actually say to bring their Certificate of Citizenship, then well, if you don't have it, you don't have it. Tell them that "I don't have it, it's not my document to have". VAWA is also one of the few cases where USCIS can do some investigating on their own, if they're a naturalized or derived citizens they can just find that by their A-number. If they had a passport they can likely pull that from the Department of State. I don't know if they investigate birth certificates with the local departments.
Moreover, USCIS does not interview for I-360s, meaning that at the time you get an interview for your I-485, whether your ex is a citizen is an established fact.