I have no idea where the water is coming from to do this. You can see the sponge that is under the passenger seatbelt tensioner. From the inside is looks fine just dirty. It's not coming from the sunroof or the washer fluid hose. The rest of the rocker panel is fine.
Maybe from the front of the running board mount where the undercoating has cracked??
The driver side has it too but not rusted through yet, and it's further forward on the rocker panel.
Driver side: