Call the council, find the planning dept and just check that. Blocking out all the light to someones place can be against the rules in some circumstances, you won't know unless you ask. If there's nothing that can be done then get on Amazon and order yourself some warm white led strip lights. Comes on a reel in 1,2,3,4 + meter lengths and it's sticky. Run that round the window on the actual brick work right next to the frame and voilà, the window becomes the light. If the window is near the door to the rest of the house then you can get motion sensor leds that turn on when you walk in. I've got them under my kitchen cabinets and they're great. Then get whatever bits you enjoy (crystals, ornaments, plants) and turn the window into a feature. If you don't like looking at the fence, get some window privacy film. There's shed loads of designs and colours available. Hell, you can get a photo quality shower curtain and staple it to the fence outside the window and give yourself a 'view' along with the lights. If you sit and think about it you can do something that looks fab with a suddenly useless window.

I'm surprised he can build it smack dab on the property line. Did he have a permit? Often, a fence must be a foot or two from the property line on their side so they can maintain both sides of the fence.