oh yeah For the fence just build the neighbours right up to his and then leave his back fence as her back fence, will save her some money. or you could just moved his to the propertyline and then charge her a bit for saving her the money from having to put in a back fence.
Leaving the back space there between the two fences will cause alot of problems with weeds and such growning up through there and will be ugly.
I know, thats why i was saying, he doesnt want his fence used as the border of her back yard, and im not sure he even has that option. As long as her two side fences do not connect to his corners, maybe if they stop 1' from his fence? He thinks that he would "lose" his 6" of property if he ever goes to sell the house .