Ransack their office drawers looking for old payslips? Not sure if there's any official way of getting that data. You could try getting in touch with their work payroll office or the local tax office.....
Might be able to find a few old payslips, but they don't always have the number on them. It's a long story why it's needed, and it hopefully may never be needed but it buy's the person who wants it a little 'insurance'
