I am listed as a reference for a former colleague (let's call him Bob). When working together at a past job (small company that had some financial troubles), Bob was unexpectedly let go for reasons that were unclear. In the months after, Bob struggled to receive his contractually due redundancy pay and even had to take the employer to court over it.
Other people were made redundant, and the company wasn't doing well, so he wasn't the only one let go and targeted for poor performance. Though in redundancies they usually keep the top performers, that's just common sense. I was not aware of any performance problems from him. However he may have been struggling on something. The company often ascribed tasks to people that weren't suited to their area of expertise. E.g. in programming getting them to fix a problem in one language when they were mainly a developer in another language.
Bob has since been looking for a new job for quite a while. Bob wants to list the former boss on his resume because the work done for that company is important to his experience and attractiveness as a potential employee.
I am also listed as a reference for Bob and when I've been contacted (not that often) I am asked unusually specific questions about whether Bob was a troubled employee at our former mutual employer.
I am concerned that the former employer is describing Bob in a falsely and exaggeratedly negative way as a form of revenge for Bob's legal efforts to obtain the redundancy pay that was due to him.
Aside from removing this former employer from the list of references, what can Bob do to ensure that his boss is fairly describing his work experience at the past employer? Can Bob take any action to prevent the employer from describing Bob negatively (given that Bob's performance at the time was never viewed negatively)?