Alterations, Revocation and Revival

A solicitor is advising a client who wishes to make changes to her existing will. The client executed her will in 2019, leaving her estate equally to her two children. Recently, her grandson was born, and she now wants to add a gift of £5,000 to him. The client suggests simply writing this addition on her will and having her neighbour sign as a witness. She tells the solicitor that her neighbour is available tomorrow and asks whether this would be sufficient to give effect to her wishes.