If you go to the paint room on the right hand cupboard, or around there you will be prompted to open a padlock - the code is 951 (can see it in the back of a painting in the room) and you will get dentist tool - then go to cylinder and use the dentist tool to get the key to open lia's door in the morning
Edit: Just to warn you for the morning, if you go to lia after opening the door and do the erotic option to fuck, your passion afterwards will be reset to 0 - one of the bugs in the game