Wrongful termination & severance accepted under duress

My husband was fired after 5 years in the company for "vague" reasons. He was the Executive VP and CFO for the company. He has been given several promotions recently and a pay raise 3 weeks ago. His boss fired him saying he did not have faith in the direction he was taking the company and he was unhappy with his performance. Yet he just gave him that raise equating to an "excellent performance" rating three weeks prior. No evaluations have ever been negative and not one word was ever said that his boss was not happy with is performance. He was given a severance package and told he either signed it that day, right then and there or he would receive no money, only that days pay. Duress? Wrongful termination? We believe the termination is due to an accidental discovery of an affair his boss is having. My husband stumbled on it 4 days prior to his firing.