They didn't do it themselves so why would it be marked as self prepared?  If you are doing it for free and are worried about the IRS tracking your income, they don't.  All of my family freebies are treated the same as paid clients.

Can you put XNP as the preparer code?

Your malpractice carrier may have an issue with this. Because self-prepared means you're not signing the returns and the 8879 or state e-file authorization forms. And if you do sign but mark it as self-prepared now you raise other concerns. Don't do it.