There was an object inside the shower (hidden or not) which the DI used to smahs the door and get out
Also some things to repeat (if you can...):
There are no way to enter or exit the shower except through the door (or a hole in the door).
And to prevent what confused everyone so much last game:
There are exactly three person related to this story: You, the DI, and the woman. When a person is referred to, there are no ambiguity on which person it is. (That is, there are no point in the story or pink truths were for example, "the woman" is used to refer to the DI)
Hope you understand what I meant, and if someone could reword it better, go ahead.