I used to do lots of these chalet bungalows from a ladder. There's a simple way to vastly improve the stability of the ladder: look at the picture, the ladder is long enough to reach the top of the dormer window, so the rubber point should be set on the wood frame above the window. The foot of the ladder can be set so that the styles don't touch the guttering without it being at too shallow an angle. When the man climbs up he will be within reach of the glass without having to overstretch, and no danger of sliding down the roof.
Cheers,
Ian