Any wet material that is left in place will simply continue to rot, and spread to any new material installed. The homeowner can most likely do this himself, without having to do any plumbing work. Take the fiberglass shower out in whatever size pieces can be managed, remove any wet insulation, and remove any other, non-structural damaged materials. There are several solutions for a proper replacement. If you say this is a double shower because this was at one time a bathtub stall, I would recommend replacing with a tub, a good water diverter for the addition of a shower head, and then tiling the wall in the same manner as described above.
Tubs are less prone to expensive leak problems.
They also have a video they will let you use. They will answer all your questions. You will be glad yo did. Remember to be very precise with getting it square and measureing everything out. Also when finished wait 2 weeks and be sure to seal your grout.