I think it's more a manufacturing fault. The opener doesn't seal properly into the frame. The frame should have a rubber seal all around it so the opening section sits into the rubber when the window is closed and forms a water tight seal. If water gets in, there is obviously a fault in the construction of the window and the householder should demand the manufacturer rectify it.
Any decent installer/manufacturer will give at least ten years insurance backed guarantee against any such faults.
On older (out of guarantee) windows, it's possible to run a thin bead of good quality silicon sealer along the inner top frame. The window must be left open for at least 24 hours to cure, and then when the window is closed it will make a water tight seal against the silicon.
If any of your custies have leaky windows with no guarantee, offer this as an 'add on' - could be worth about twenty quid a window and take you all of five minutes to do
