Help or suggestions please

我老婆在一公司9年. 前後跟過4個老板, 前邊3個都可以, 每次評估得個人2.3 到2.7 (2 means meet and 3 is the highest performance).最後一個老板對她不好 (together in two years), 也可以說合不來,this time 給她一個不好review (1.7), means it did not meet requirement and let her go. she talked to the big boss and did not get any better. if take legal action, any chance to win? any recommendation and suggestions are appreciated.