Thank you very much for clarifying. First of all, I am sorry to hear about the issues she has experienced in her situation.
If an employee has been continuously employed with their employer for at least 2 years they will be legally protected against unfair dismissal. This means that to fairly dismiss them the employer has to show that there was a potentially fair reason for dismissal and that a fair dismissal procedure was followed.
According to the Employment Rights Act 1996 there are five separate reasons that an employer could rely on to show that a dismissal was fair: conduct, capability, redundancy, illegality or some other substantial reason (SOSR).
Therefore, the first step is to establish which of these reasons is going to be used to justify the dismissal. Not only that, but they must also justify that it was appropriate and reasonable to use it in the circumstances.
Finally, they need to ensure that a fair dismissal procedure was followed and that the outcome was one that a reasonable employer would have come to in the circumstances. Each of the potentially fair reasons will have its own requirements in terms of the procedure that needs to be followed in order to make it fair.
They are also expected to honour any notice period in her contract, unless she is being dismissed for gross misconduct, which would not be the case if it was a performance issue only.
If she wanted to take this further, she should appeal with the employer first and if that is unsuccessful, she can contact ACAS to initiate the unfair dismissal claims process and seek compensation.