I think some of you may have missed the part about him already removing the rotten wood from underneath.
Dry fit a piece of 3/4 marine ply. Pre-install the desired number of layers of fiberglass on the side that will be facing down. Make a T shaped brace somewhat smaller than your plywood. 2x4's will work fine. Get a couple of small jacks or other suitable items to hold everything in place. Pre-wet the top side of the ply and the bottom of the sole with a roller. Using epoxy here will be an advantage because it gives you time before it kicks. Mix epoxy and cabosil to a mayonaise to peanut butter consistency. Trowel it on with a plastic 1/4 knotch trowel as suggested earlier. Brace it in place. Glass in the edges with the desired layers of cloth strips.
After it dries overnight, remove braces and reinstall any removed hardware. Sail.
This project could easily be completed from the point you're at in a weekend.