I want to stress that the fact that she is not receiving payslips from her employer does not automatically make her self-employed. To be self employed, you would be in control of what you do, when you do it, where and how you do it. You need to be able to hire or fire people on the same project etc.....
If none of the above is true, then she is an employee who did not receive any paylips from her employer. Therefore when filing a return, she would just look at her bank statments and report the amount she received on her return.
She does not need to attach any proof of the income until when /if the return is audited by the IRS and State authorities. Therefore the back up / bank statments should be kept with a copy of the return in a safe place for 6 years. The only documents required to be attached to the US returns are W-2s and 1099s or any other documents showing Federal / State withholdings.
Helen