Doors and Windows - bow window

Me and my father are remodling his house, and we are installing a bow window. In the process, we took off the old alluminum siding, and cellulose board. We replaced it with foil backed R-Board. We have the opening framed up,and now we are going to fasten the window to the frame. The window has nailing fins on the sides, and we are going to build a roof for the top. Now, here is our delema. My dad wants to take the R-board off the sides of the windows,and replace it with 1/2 inch plywood, to make a solid back for the nailing fin. I say we keep the R-board and use 8x3 galvanized screws through the R-board and the 2x4 framing, to provide a water tight seal under the nailing fin. He insist that the R-Board is to squishy and will eventually work its way out, and the window will fall out! What do you think is the right way?

The right way would to mount the window directly to the framing and run the R-board over the nail fins.