Unless she has got written agreement to shorten the notice period then there is not much she can do. Of course her new employers might not care.
here is what ACAS says
If the employee leaves their job without giving the proper notice, the employee may be in breach of contract.
The first thing the employer can do is to discuss this with the employee; they may not be aware of their duty to give notice, or how much notice they need to give.
If the employee has resigned without giving notice because they believe that they have been constructively dismissed, the employer should encourage the employee to raise a grievance so that their concerns can be investigated properly.
If the issue cannot be resolved and the employer wishes to take this further, they may make a claim for breach of contract through the county court. The remedy would be compensation for the net financial loss that the breach of contract has caused. (For example, this might be additional staffing costs the employer has incurred, set off against the employee's wages that they have not had to pay for the notice period.)
Alternatively, if the employee has made a tribunal claim against the employer for breach of contract, the employer could submit a tribunal counter-claim.
In the meantime, the employee is entitled to wages for the period that they have worked, as well as any accrued holiday pay until the end of their employment.