Most don't know (although I've known it happen, when the alternative was more redundancies).
But the net pay number on the emoyees pay slip will be the same after that first year.
With a few simplifications:
The employer makes £income extra money from that employee working for them, so they can't have their total cost of employing them exceed that £income.
With say 4% inflation (assuming it's even - which is a simplification) they'll recieve £1.04income extra with the employee working for them
So they can now increase the salary offered by 4% as well to £1.04salary, in order to maintain the emoyees purchasing power (and because all other employers will, so they need to in order to keep the staff)
If a 2% NI rise is levied on the employee, then the employe gets £1.02salary net
If the 2% NI rise is instead levied on the employer, then instead of giving the 4% rise, they give only 2% rise (because their cost has gone up 2% - as have all the other companies they compete with for staff). The employee doesn't have the NI levied, but their gross is still only £1.02 salary.
After a single year, the net amount in their payslip is exactly the same either way.