I would have the ex report the income on Schedule C.  Then on Part V of Schedule C I would enter that same amount with the description  "Amount Properly Reportable by Taxpayer A, SSN xxx-xx-xxxx.  I would also attach a Word document to the return to more fully explain what happened.

If you strongly believe that a Schedule C shouldn't be used because the ex does not have a business, then the alternative would be reporting it as other income and then subtracting the same amount on the other income line with the same description, again attaching a Word document to fully explain what happened.

The point is regardless whether you put it on Schedule C or on the other income line, the ex must report this to avoid IRS correspondence later on.  I used to not get too worked up about the possibility of IRS correspondence but given it can take a year or more to resolve an issue these days, if there is the slightest thought in my mind that IRS can question something remotely unusual, I attach detailed explanations.