Family leave
A. No. Because of the unequal bargaining power between employers and employees and the risk that employers will put pressure on employees to give up their rights, employment law has special rules preventing employees contracting out of their statutory employment rights. These include unfair dismissal rights and rights under the Equality Act 2010.
If your member fails to return on the agreed date, she will be at risk of dismissal, but your employer must carry out any dismissal fairly. Otherwise your member will have a potential claim for unfair dismissal.
Any agreement by your member that failure to return on the promised date will automatically end the employment contract will be of no legal effect whatsoever. It makes no difference that the agreement is written down and signed.
The only valid way to give up rights is through conciliation using a conciliation officer employed by the Acas Advisory Conciliation and Arbitration Service, or by entering into a valid compromise agreement (renamed Settlement Agreements from 29 July 2013). Any other agreement by an employee to give up these rights would be void.