When doing rough plumbing “before the cement slab is poured…”
the plumber will rough in a trap and waste arm to the center of where the shower drain is supposed to go and box out or make several wraps with styrofoam wrap so that a small void area around the trap riser is created so that the poured concrete does not get too close to the trap riser to accomodate the bell shape of the fiberglass shower drain on finish.
After the slab is poured and the plumber comes to set finish, he can remove the dirt or styrofoam easily from the finished concrete to expose the trap riser.
The fiberglass shower pan is then placed over this riser and a rubber gasket is then packed down around the trap riser from the top of the fiberglass pan creating a watertight seal around the pipe and the fiberglass shower.
Any excess of pipe sticking above the actuall rubber gasket is then removed with an inside wheel cutter(ABS installations)to avoid any fouling areas.
Hopefully your gasket is just damaged or for some reason missing,,,if its your trap that is rusted out or cracked, then your basically screwed! Youll have to remove the whole shower pan and replace the trap!
You can look to see if your trap is holding water by removing the cover your husband caulked in place and shining a flashlight down the drain.
If you dont see the reflection of any water in the trap then you have problems!
I recommend you spend a couple bucks and just have a competant plumber come over and check this out for you.
Also check your policy to see what is covered and what is not…if you just bought the place you shouldnt have to worry about such things yourself!