The American’s with Disabilities Act (ADA) is a very complex law which can be challenging for an employer to understand.
One of the requirements of the ADA is that an employer can not discriminate against an employee because of their ‘association’ with a person who has a known disability. The thinking here is that an employer will not think kindly to a person possibly taking time away from their job to assist someone who has a ‘known’ disability.
This article outlines in more detail this ‘association’ element of the ADA.