For the financial requirement, Category A, your sponsor will need:
* Six FULL months of payslips (this may mean seven payslips to cover the six full months)
* Six FULL months of bank statements covering the same span of time as the payslips and showing the salary/wage paying in
* A letter from the employer confirming the employment, title, salary, when that began, and whether it's permanent or temporary, etc.
* Optional but recommended: the employment contract
* Optional but recommended: the latest P60 if available (though in your sponsor's case, this might not be helpful, and it's not a required document)
Note: The most recent payslip, the most recent bank statement, and the employer letter must all be dated no more than 28 days before the date you apply. So don't get that letter yet.