What should the instructor do about a student who is not aware of their disability?
Depending on the nature of the disability, I'd suggest first talking to the relevant disabilities/accommodation office. First, they may have some paperwork or data about the student which hasn't been given to you; I've had students with accommodations before who were simply uncomfortable or unable to present them to the instructor. Second, even if they don't, they'll have more guidance on what to do here which may be very school specific.