This is a classic case of a bad plumbing connection behind the wall.
Every time the shower was used a small amount of water would leak into the gib etc.
Overtime the outcome in this case was a nightmare – the poor workmanship ended up causing massive failure in building integrity, rotting structural walls and bottom plates.
Black mold is worse than a pee ridden house – trying to do something without employing an expert is totally avoidable – choose the right plumber for the job and you will avoid situations like this.
If you ever have an issue act fast to control the situation – pick the right plumber in the first place and you won’t get in trouble. Imagine explaining this to your family if it was the result of a cheap installation – that’s code for you bought it cheap and tried to do it yourself!
And now you wish you hadn’t…