Can a line manager alter a timesheet afer being submitted by an employee?

Basic working hours are paid at a flat rate for the month. Any unsocial hours must be submitted via timesheet to the line manager and then sent to payroll.

The line manager had not informed the employee that there were errors, or that there were areas of concern on the timesheet. The line manager had simply crossed out the hours and handwritten less hours onto the timesheet, then submitted to payroll.

The employee only became aware after their unsocial hours were not as they expected and queried payroll. Payroll forwarded a copy of the timesheet. and it shows that the line manager had crossed out the hours, signed it, dated it, then submitted to payroll.

The employee is not happy and wants to know how this has happened without them knowing.

Any advice would be gratefully received.