I would warn him of the violation and ask him to take a LEGAL stance.
You are not allowed to back into branches to move them out of the way. You must attempt to take a stance that results in the least moving of branches and the like.
803.05 Obstacles and Relief
A. Obstacles to a Stance or Throwing Motion: Players must choose a stance which results in the least movement of any part of any obstacle except as allowed for casual obstacles by 803.05 C. No relief is granted from park equipment (such as signs, trash cans, picnic tables, etc.) as they are considered part of the course. Once a legal stance is taken, a player may not move an obstacle (or hold it back or bend it) in order to make room for a throwing motion. It is legal for a player’s throwing motion to make incidental movement of an obstacle.
Z-Pred, Z-Force, Destroyer, (M,S,Q)OLF, Roc, Wizard&VP