Evon,
This more of a math question... Depends on what length do you want the right and left halves of the V to be?
It would be as simple to just take the Y coordinate of the mouse click and add lets say 50 meters and then take the X and add 25 meters and create a new Point class from those coordinates to get the right side and then do the opposite subtracting from the X 25 meters and adding to the Y 50. Now you have three points your left point your mouse click and your right point. now you create a Polyline from the points:
polyline.addPath([new Point(evt.mapPoint.x - 25, evt.mapPoint.y + 50), evt.mapPoint, new Point(evt.mapPoint.x + 25, evt.mapPoint.y + 50)]);