Co-owners on a house, not married, both on 1098

What is the legal way to file single unmarried, joint homeowners. Both of them are on the 1098. She is listed first with SS# and makes less and would like to do the standard deduction.  He would like to take the entire deduction to itemize and is listed 2nd on 1098.  All house bills are paid from a joint account that they each contribute to to cover all household expenses including food and utilities. Can he claim the entire house if she is listed first with SS#? Some articles say it must be a 50/50 split. Thanks for the help.