Its not in the rules because stacking does not affect facing.
Facing is not relevant to CC. Perhaps he meant short range. The only possible relevancy would be cost to "exit" out of a CC, because certain hexes may not allow exit:
If the defending unit decides to retreat out of the hex, it may not move into the same hex or either of the two adjacent hexes from which the advancing unit had just moved from. It may move into these hexes in later actions.