Why does Facher say these people can never testify in the film civil action?

In the film "A Civil Action," Facher argues that certain witnesses cannot testify because they lack credibility or have conflicts of interest that undermine their reliability. He emphasizes that their testimonies could be discredited due to personal biases or a lack of firsthand knowledge about the events in question. This strategic decision highlights the challenges faced by the plaintiffs in proving their case against powerful corporate defendants.