Most likely the Help desk in this case was not very helpful.  IRS does not pre-determine that the first to file is the rightful owner of the tax credits.  It only takes about 4-6 weeks by mail to get refund.  So I would file by paper with correct info including reason having to file paper form.  Then both parties usually get a letter about October with instructions to file 1040X if they claimed the dependent in error or if they need proof they are the rightful claiment.