Cats learn their entire life, so please do not worry about the litterbox. Most likely you will only need to show her the litterbox and how to dig a little.
To show her how to dig, you simply put the cat onto the litter and take her paw and dig a little. You will need to take the lid off the litterbox and leave if/or until she understands what you want.
The thing you want to do is to put the litterbox in a quiet place of your house, so she can do her stuff undisturbed.
If the cat has problems finding the new litterbox, she will let you know. You might want to leave the lid off the litterbox for the first day or so, until she starts to use it.
It might be best if you close the shower or she might use it, I find it strange that the owner lets her use the shower as a toilet.
