If what they say has merit, then it can be held against him. It will be up to the judge/jury to decide what value to give to the statements. It will be up to his attorney to prove they are liars.
Neither is an easy task. Experienced counsel can make a difference. You should consult with his attorney on what efforts are being made to prove that the people are liars. And, if not satisfied with his answer, consider retaining experienced defense counsel.