How are disabled passengers evacuated during emergency?
You can find the full text of the FAA's recommendations in this AC

But the flight attendant is responsible for discussing with them whats best.  


  ASSISTING DISABLED PERSONS. The handicapped person knows best . the
  manner in wbch he can be assisted, and flight attendants should not
  hesitate to discuss this with the individual. Keep in mind that
  different disabilities have different needs, and injury can result if
  a person with a disability is improperly assisted.


The AC goes on to describe most major scenarios and common instances.