I'd take that to court tbh - pretty sure they can't pull a stunt like that without proving she was underperforming or causing difficulties for the firm. Name and shame the gits
Unfortunately UK employment law is pretty weak, especially if you don't meet the current 2 year qualifying period for service commencing since 6 April 2012.
However, there are some exceptions where you don't need the qualifying service period. For example if she can demonstrate that the dismissal was really due to discrimination (age, disability, gender reassignment, marriage and civil partnership, pregnancy and maternity, race, religion or belief, sex and sexual orientation), whistleblowing, trade union or health and safety activity or asserting a statutory right.
Disclaimer: IANAL