Well, I assume he had a leader in the Assault Moving stack ( or the leader was moving alone) to allow 6 movement points which is more than the 5 required. If the target hex was basic jungle (ie no smoke, wall...) then the movement points would only be 2 points for placement and the total 4 points instead of 5.
Anyway, you can Defensive First Fire (DFF) upon any of the movement points. You can also DFF just after he states his unit/stack has stoppped expending movement points. This last point was a "Perry sez" clarification.
For example, you can't move three hexes, say you are done moving that unit and avoid DFF against that moving unit.
Rich