In this instance, yes. Marsha may ask Walter whether a medical condition is affecting his ability to perform the essential functions of his job. Generally, the Americans with Disabilities Act (ADA) allows an employer to ask disability-related questions or require an employee to have a medical examination when it knows about a particular employee's medical condition, has observed performance problems, and reasonably believes that the problems are related to the medical condition. At other times, an employer may ask for medical information when it has observed symptoms, such as extreme fatigue or irritability, or has received reliable information from someone else (such as a family member or coworker) indicating that the employee may have a medical condition that is causing performance problems. Often, however, an employee's poor job performance is unrelated to a medical condition and generally should be handled in accordance with an employer's existing policies concerning performance.

Source: EEOC Publication: Revised Questions and Answers about Epilepsy in the Workplace and the Americans with Disabilities Act, http://www.eeoc.gov/laws/types/epilepsy.cfm; reported in Employment Practices Guide, New Developments ¶5373.