Answer: If the grandparent is not the legal guardian, then the student is neither independent nor should provide the grandparent’s information. The parents’ information would still be required. The way to navigate this situation is to seek a “dependency override” from the financial aid office of the school she wishes to attend. This involves explaining the situation and asking the financial aid administrator to make a discretionary determination of independence.
A good summary of independent students is available here: https://studentaid.ed.gov/sa/fafsa/filling-out/dependency#dependency-questions