We had our friend check it out (expert) and they found
rot well into the floor, main beams, and into the floor
and wall--even into the next room! Be prepared to gut
the entire bathroom. You don't want to just do patchwork because
the rotting is a live thing--it spreads. You have to keep removing
wood until the rotten wood is gone. We are now at the stage of
putting new wood back in and reframing the shower after some
plumbing repairs that we saw needed to be done, too.

We're going to use a one-pc. shower that we haven't picked out
yet--but it will be seamless! The seams are what will cause you
grief later. We estimate that this will cost over $6000--but we are
getting a deal because we are family/friends w/ the guys doing
the work. Good luck! You should have it checked out soon or the whole
thing could cave in--or worse, the spreading will enter your bedrooms
or living room!