She does need to use a parent’s information. There is a decision tree of which parent to use. First is which parent does she live with more (neither). Then it is which parent provides more support (neither). Then it is which parent makes more money (??) If it is easier to use the mother, to get her signature, do that.
As kelsmom says, this is a case for professional judgment. The school should be able to help you file the initial FAFSA and then override it.