A cavalry without an attached leader may Retire and Reform one hex onto a hex that contains an unattached friendly leader (i.e. a leader alone in a hex). The leader is immediately attached to the unit and the cavalry unit does not have to retire and reform its second hex.
In fact, shouldnt the cavalry unit be forced to stop on the leader and remain? The wording says "may" but in other leader rules, units must stop when they move to a space with lone leader.
A unit may move onto a hex occupied by a friendly leader when the leader block is alone in a hex. The unit must stop and move no further on that turn. The leader is considered attached to the unit.