When my husband takes a shower upstairs, it leaks on the basement ceiling. How do we fix it?

when my husband takes a shower upstairs, it leaks on the basement ceiling. How do we fix it?l you have to determine if this is a leak from the mixing valve or the drainage. First remove the knob and face plate from the mixing valve. Is it wet behind there after hubby showers? No? Then open up the ceiling under the trap and check it while he's taking a shower? If the trap's dry then check the tile grout where the shower stream hits. Get back to me with the results. Good luck, Tom