I am definitely shopping for a shrubbery/plant barrier for this summer for sure, it's just a matter of figuring out what will grow best/fastest in that gap. It creeps me out that he has nothing better to do than stand in his backyard taking pictures of mine.
Bamboo is the fastest growing, but check your state laws on what varieties are legal most states have several varieties listed as noxious weeds or invasive species... Also consider 'clumping' bamboo vs 'running' or else you will be overrun... Or if you like to rub it in as some people just can never be pleased or dealt with, rent a trencher and cutting an 18" deep trench about 1 foot on your side of the property line and drop in some 24" aluminum flashing to create a wall 18" under ground and about 6" above ground (edge with landscape timbers to hide aluminum) then plant running bamboo between that barrier wall and your fence, sit back and enjoy the show as the bamboo runs into his yard...
Butterfly bushes are another fast growing option, I had considered tilling under all the grass in my front yard and planting only butterfly bushes when I lived in a neighborhood with a busy body neighbor liked reporting everyone that dared skip a week of mowing...