Correct. The net worth of the rental house (current market value minus any outstanding mortgage on the house) needs to be added to any other reportable assets that the parents have.
The rental income should already be included on the tax return, if they had rental income in 2015 (2017/18 FAFSA), and reported it.
The FAFSA should be done together if possible. It’s the student’s application for aid, but parent help might be needed for income and asset information. And one parent has to sign along with the student.