Yes. An employer may request reasonable documentation where a disability or the need for reasonable accommodation is not known or obvious. However, the employer is entitled only to documentation sufficient to establish that the employee has diabetes and to explain why an accommodation is needed. A request for the employee’s entire medical record, for example, would be inappropriate as it likely would include information about conditions other than the employee's diabetes.
In this instance, a note from Joey’s doctor would be sufficient to show that he has a disability and needs the requested reasonable accommodation. If Joey makes a subsequent accommodation request related to his diabetes (for example, he asks for a shift change) and the need for accommodation is not obvious, you (as his employer) may ask for documentation explaining why the new accommodation is needed but may not ask for documentation concerning his diabetes diagnosis.
Source: EEOC Guidance: “Questions and Answers about Diabetes in the Workplace and the Americans with Disabilities Act,” http://www.eeoc.gov/laws/types/diabetes.cfm; reported in Accommodating Disabilities Business Management Guide ¶140,325.