Other than all the obvious with this repair, you have an advantage that you can get at it easily.
I would start by pushing from the inside and open up the crack and put epoxy on the edges of the crack and then use weights and push it back in shape with the epoxy bonding the crack together. Do a dry run first. You can also take small pieces of wood on the inside that bridge across the crack and screw into them from the outside holding the crack together. Use wax paper to keep from bonding the support blocks. Once you have the crack bonded together, then its the usual routine with sanding and a few layers of glass inside and out.
If you know the owner, perhaps he/she has left over paint.