You don't use a Line item, because putting this on the invoice is the same as "this is paid for the date of the invoice" and not Paid on a different date. Factually, the Payment is its own transaction and has its own date.

You should show it on the Footer; not adding it as a line item. You also can run a Statement for them.

"but it does not show on her invoice only on mine"

You need to Customize your template(s).