For the purposes of filling out the FAFSA, divorce and separation are treated the same. Whether parents are legally divorced or are just separated but not yet officially divorced, only one parent needs to report income and assets on the FAFSA. A separation, for the purposes of the FAFSA, is defined as parents whose relationship has ended and who maintain separate living arrangements. Living in separate bedrooms in the same house does not count, nor does a parent living temporarily in a hotel room or with friends or relatives.
The custodial parent is the one who should fill out the FAFSA. This is not necessarily the parent who has legal custody, but rather the one who the student has actually lived with most during the 12 months prior to submitting the FAFSA. If a student has lived with both parents equally, then the one who provides more of the student's financial support is the custodial parent. If both parents provided equal support last year, continue looking back until you come to a year when one parent provided more support. Other criteria that can determine the custodial parent if all else is equal include the parent who has legal custody, the parent who claimed the child as a dependent on the tax return or the parent with higher income.
If the custodial parent is remarried, the stepparent is treated as a parent on the FAFSA. Therefore, if a student's mom is the custodial parent and she has remarried, any question on the FAFSA that asks about the student's father needs to contain information about the student's stepfather. This is true even if the student's mother had a prenuptial agreement that precluded the stepfather from being responsible for paying for the student's college education. The federal government does not recognize prenuptial agreements in this case.
The FAFSA does not ask for any information about the non-custodial parent, beyond what the custodial parent reports as child support received. However, some schools, especially expensive private schools, might require students to submit financial information about the non-custodial parent. This information might affect the institutional grants the student receives, but it will not affect federal and state financial aid.