In reply to Joe Friedman:
The simplest harness would be made out of a pillow and a pillow cover to look like helicopter rescue harness pictured here:
http://www.atlasreed.co.uk/resources/Rescue-Harness-Irvin-GQ.jpg
Couldn't you throw a rope across the fence/hedge/wall, surrounding the house and in the night sneak out to fetch it? Your idea suggest he'll have to sneak out of the house anyway...
Another option: does he need to do the climbing thing? He could lower himself off from above her room window on a rope made out of his sheets. There could be a roof window or some "high altitude" climbing action involved with going up onto the roof from his room. Or a bathroom?
Getting down is simple on the rope made of the sheets as mentioned by someone else (his and hers combined).
She could be lowered in the helicopter harness tied on one end of the rope - the other would be wrapped couple of times around a radiator to give sufficient friction so the hero is able to hold her weight.
He could then use it in the way showed in this video:
If he can leave the rope there (to be discovered and engage a pursuit) he could simply tie one end to the radiator and do the classic abseil technique (first in the video) on a single piece of rope. Otherwise he'll need length of twice the height - you see why?
I can see the romantic idea of him carrying her down but that, done with the classic abseil technique, would be extremely painful if not impossible (need to try...). That said - the pain could make it more "romantic" and having sheets instead of a rope could give more friction and softer touch. And it's not real story so you could improve the reality a bit...
Please do send me the copy of the chapter for corrections. I don't know how a book looks before rewriting but I suppose I'd get the general idea and the important climbing bits.
Should you want to try something out outside - our East Grinstead Climbing Club (to east from Gatwick) would surely be excited to do some action on Southern Sandstone or any houses around
Just bring the sheets along.