You can go to the myStudentAid app and there will be an option under a list of special circumstances to check if your parents refuses to provide the information needed.
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?