The DI left the shower first through a secret door, got a tool, and then returned to the shower to break the glass dor. He then left through the hole.
The DI took the water hose and broke the wall with it.
The DI bit himself, waited for the blood to coagulate, and then broke the door with the blood chunk. He then killed the woman with it.
Also, a question if you want to answer: The trick the DI used to escape the shower, is this a trick that can be used without preparation in 'normal showers' as well (as long as they are the same design) or is it something specific with this shower/this occasion that makes the trick only be usable there. (like a secret door, things already placed in water, etc.)