Restroom (Hotel Shops)
From the Super Mario Wiki
The Restroom is a location in Luigi's Mansion 3. It is found in the Hotel Shops area on the third floor of The Last Resort. It is a restroom found on the first floor of the Hotel Shops, to the very right near the Elevator Hall. The women's restroom is blocked off by a red rope, so Luigi is only able to enter the men's restroom.
Although Luigi cannot explore the women's restroom, there is a drain in the northwest corner of the men's restroom which Gooigi can enter to get to the opposite restroom.
In both restrooms, there are stalls, sinks, dryers, and garbage bins, making them almost symmetrical. The purple gem is located on a toilet in the women's restroom, but the toilet is overflowing, so Gooigi is not able to get to it. He must first use the Dark-Light Device to reveal a drain on the floor of the men's restroom which can be seen through the mirror. Gooigi then needs to fall through and head left to get to a valve, which he must turn to stop the water flow in the women's restroom. He is then able to go back and get the gem.