It doesn’t matter which parent takes the tax deduction because the IRS allows divorced parents to swap them back and forth. Often one takes them in odd years, the other in even. The IRS rule is not the same as the FAFSA rule.
For FAFSA, just be honest. If time is truly equal, ex can claim them if he supplies slightly more support. In fact, he can claim all three because that’s what the rules say. Next year, he could do the same if he’s filing out FAFSA.