Child support is not taxable so it does not show up in the FAFSA linkage to the IRS. However, it is reported as untaxable income on the CSS Profile (and FAFSA- edited to add).
Since the current child support paid by you to the mother, is for the son, the Special Circumstances section of the CSS Profile may be used to explain that child support for the daughter has ended and that that portion of the income is for the son.
I do not think your ex-wife is supposed to use the child support for the son, to pay for expenses for your daughter.
Here is another thread on this: CSS Profile, Child Support/Alimony - Financial Aid & Scholarships - College Confidential Forums - Admission Forum Discussions and Threads
I would not assume fraud. Many families have these estrangements and some heal them, some don’t. It takes work and possibly counseling.