Can my husbands employer fire him if an employee lied about him?

Long story short my husband is a manager for a large company. He was called into the regional managers office because during an investigation into a couple employees who are thought to be doing and/or selling drugs, they retaliated saying my husband illegally bought a gun from another employee (which if you knew my husband is laughable and utterly ridiculous). He is worried he can lose his job since more then one employee made the complaint and there is no "proof" if he did or did not do what they are saying he did. I understand the employer still needs to investigate and ask questions, but could his job terminate him for what is obviously (at least to me) retaliation by his employees? And if they do terminate him what is his legal recourse, if he has any?