I think it would be easy to prove Walmart’s negligence (or innocence). If an employee told a manager that he (the employee) had virus symptoms, those folks who worked near that employee should have been notified.
Just tell the truth. The employee had symptoms. That doesn’t mean he has the virus. Maybe he just has a cold. But don’t keep anything secret. Let the other employees know, then let them decide for themselves what to do next.
I think it’d be easier to prove Walmart at fault if more than one employee from that store also contracted COVID.