Is your former employer allowed to contact your current employer to ruin your life?

Suppose an employee has a job, we'll call it Job A. This is a full-time 9-5 office job the employee absolutely loves. In Job A, the employee gets along with his coworkers and the manager is very nice, but the pay is not enough to make ends meet, so he gets Job B, a part-time weekend and evening fast food/retail gig to make extra income. The problem with Job B is that the manager is an tyrannical asshole who rules with an iron fist and micromanages the employees. This employee is not having it, so he gets the balls to sign out of the register that still has a line of customers, slam down his badge, tells this evil manager to go f--k himself while flipping the bird, then proceeds to clock out without putting in a two-week's notice. The employee doesn't care since he already has Job A. The enraged manager threatens to contact Job A about the employee's shenanigans at Job B to get him fired from Job A while manically laughing as the employee exits the door. In the event the employee wants to search for a new job, he will never list Job B on his resume. Can a former employer contact your current employer to get you fired, or are they legally required to maintain confidentiality by not slandering former employees?