my friend lives in a second floor flat and has arranged with the ground floor that she can put a descreet plant of wood from her window to the ground, the cat, as cats are, is very agile and just runs down the pank when he wants to get out, you may have to put two planks together, but it only needs to be a 6x2 and she/he will be find to get to the ground and back with the window open a crack, may just want to put a tie on the window to a certain width or screw the window with a piece of wood a few inches thic,.