ADA may require a reasonable accommodation. The employee has obviously asked for one. Since the employee's condition isn't obvious to you, the next step is to determine whether the condition is a disability under
ADA (you can ask for documentation of the existence of a disability and why a reasonable accommodation is needed). If it is and the employee is qualified you would be required to provide a reasonable accommodation because an ergonomic back cushion wouldn't qualify as an undue hardship.
ADA was recently revised so make sure your information on the law is up to date. More impairments qualify as a disability under the revised
ADA.