Just tell him it is too late and the wheels of progress are moving on.  He has to go home to more ass chewings, maybe he will get some cajones and kick her out on the porch.  I would not let him make changes after agreements were reached though.

Second choice is buy him out and tell him he can take his little love bird and hike it on down the road.

These solutions are easy for all of us as we are not you.  That little lady that has him by the got-dang-it is why family farm situations wind up in a going out of business auction.