This seems to be two separate questions - one on how the parent should complete the form, and 2nd how the adult child can relinquish their permanent resident status. The adult child needs to discuss with an immigration attorney how to best handle thier options if they no longer wish to maintain permanent residence.
If the person has an A# they must list it.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.