As with consultants, counselors, and mentors, a discipler has considerable experience from which to draw in helping persons make desired change. A Christian who disciples is mature in the faith and is able to impart that faith to others - to instruct them in the tenets of our faith, its moral code, spiritual disciplines of the faith, and the like.
Discipleship is tremendously helpful, and coaching in no way replaces it, but the mode and methods of people-helping are dramatically different.
Discipleship is more of an "outside-in" model of people-helping versus the "inside-out" of coaching. Coaching is non-directive, non-advisory, non-instructional. It is inquisitive, exploratory, genuinely curious.
A coach could not possibly be an "expert" on the things around which they coach because no one could be an expert on someone else's future. No one could know what someone else's life path should be; what their core values are, their sense of life purpose or calling, their unique giftings, their passions, etc.
Clients are often not aware of these things either, but the assumption of coaching is that the answers they seek are to be found within themselves and their individual walk with God.
Romans 11:29 states "…God’s gifts and his call are irrevocable." It doesn't say we will necessarily discern those gifts and calling, nor fulfill them – it just says they're inside of us and they're not going to be taken back. A Christian coach helps clients discern and then fulfill what God has uniquely placed within them.
Proverbs 20:5 captures this beautifully: "The purposes of a man's heart are deep waters, but a man of understanding draws them out."