The ADA prohibits discrimination based on relationship or association in order to protect individuals from actions based on assumptions that their relationship to a person with a disability would affect their job performance, and from actions caused by bias or misinformation concerning certain disabilities. For example, this provision would protect a person whose child has a disability from being denied employment because of an employer's assumption that the applicant would be unreliable, using excessive leave, to care for the child. Employees may still be held to the same conduct and standards as all others, and can be expected to complete work as usual. The employer is not obligated to provide reasonable accommodations related to the child, given that it is not the employee with the disability.
The relationship or association provision also protects an individual who is affiliated with a specific disability-related organization For example, if the employee volunteers at an HIV clinic, they would be protected under the ADA from a discriminatory employment action motivated by that relationship or association with the volunteer organization.
For additional information, take a look at the following resources:
FAQ: Who is protected from employment discrimination?
FAQ: What is the definition of disability under the ADA?