Coaching should include everything that in service of the client and will best serve the client. A lot of times that requires much more then asking questions and validating someone.
A great life coach can definitely help a low-functioning person if that low-functioning person wants to change enough. A good life coach will not allow the low-functioning person to make excuses. The life coach will be determine to call the person on their excuses over and over again. I don't even believe there is such thing as low-functioning. Just a person who is not motivated in certain areas of their life. I am sure there is other areas that they are higher functioning. If they care about something you'd be amazed at how functioning they are. The job of the life coach would be to help explore with the person what the supposed low-functioning person really cares about and how to set up their environment to support them to move forward in the area of the life they really want.
Coaching does deal with goals, getting clear on your objects, and moving forward in your life.