The effect of addition of helium gas to the following reaction in equilibrium state, is:
${\mathrm{PCl}}_5\left(\mathrm{g}\right)\rightleftharpoons {\mathrm{PCl}}_3\left(\mathrm{g}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)$

Addition of inert gas to the equilibrium at constant pressure will increase the volume of equilibrium mixture. In order to restore the equilibrium pressure more gas molecules are required. ${\mathrm{PCl}}_5\left(\mathrm{g}\right)\rightleftharpoons {\mathrm{PCl}}_3\left(\mathrm{g}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)$
So in the given reaction equilibrium shift in forward direction to produce more Cl₂ and PCl₃ gases. (addition of inert gas to the equilibrium mixture at constant volume has no effect on the position of equilibrium)