If the central locking stopped working after she replaced the battery it is possible you may have to resynch the key to the car as the radio frequency may have reset. Follow the below steps or look up resynch my key in the owners handbook :-
1- Insert key into ignition
2- Turn key to position 2 (radio on engine off)
3- Press and hold any button for a slow 5 count
4- Remove key from ignition and try buttons
PS DONT go to a dealership if you do end up replacing the key. Other businesses will charge upto 70% less.
Good Luck