Can a coworker be sued for defamation/slander

My husband was recently terminated from a job he had held for 25 years. He was one of the top earning sales people in the company. So, it would be unlikely that they would terminate him over something frivolous. He was terminated because a coworker reported to management that he said something inappropriate to a female business associate. The associate has been a long time friend of mine and my husband and what he said, whereas may have been considered inappropriate in other contexts, was said jokingly between friends. The associate even called my husband's boss and told him that he was kidding around and she was not in any way offended. The termination letter to my husband claimed that an investigation had been done and that the charges against him had been found to be true. Based on what little we know, in as much as the employer refuses to give any information about the alleged investigation, the story the coworker told was likely greatly exaggerated in order to get him fired. Besides my husband, the associate, and the coworker, there were 3 others present. None of the 3, my husband or the associate, were interviewed. Is there any way that the company can be compelled to reveal exactly what the coworker said? And, in that the embellished story got him fired, would my husband be able to sue the coworker for defamation/slander? Not only was my husband fired, his contract included a 3 year non-compete, so he basically cannot work in his profession for 3 years. And, we also immediately lost our health insurance.