My friend is also filling out a form and wants to know what to do if their parents were never married, if that makes any difference.
They would still be your friend's parents regardless of marital status, so your friend should still try to fill out the form with their help.
You said there were special circumstances to not filling out the parent part. What are they?
If your parents are incarcerated, you have left the house due to abuse, you don't know where your parents are and you cannot contact them even though you are not adopted, or if you are between 21 and 24 and not living with family but homeless or at risk of homelessness, those are the special circumstances.