Working off ladders is a very dangerous business,my respects to the man's family,there are many devices you can buy to stop ladders slipping,the best thing apart from paying a ladder footer, is a 10ft piece of scaffold board to put ladders on if the ground is slippery,if your really high and worried you can use window suckers and a ratchet strap,this will also make things a little safer for you. p.s my ladder has no rubber feet so it digs into the board,if I can without doing damage I sometimes knock a couple of nails in to make sure the board cant go anywhere.
This will not stop you falling off a roof of bay window.
Be as safe as you can ,always make sure you work safely and sensibly.