If there was rust on his motherboard... he hasn't been drying/storing his equipment properly. DigitalRev completely submerged theirs... and it still worked after being dried out.
Also, if you're working in the rain and your camera is getting wet and you get an error, you do not try to re-power your camera. Your Camera doesn't know that it's likely water causing the error, you as the photographer should have enough sense to figure that out. You power down the camera, remove the battery, towel dry the exterior and let it dry out. Putting your camera in a sealed storage box with Molecular Sieve packets
is the best and probably quickest way to dry out your camera. An air-conditioned room, or a room with a dehumidifier running is also an option, but depending on how low you can drop the humidity level, it may take a few days to dry it out completely. Placing it in a brown paper bag then placing it in a tub of uncooked rice may also help draw out some excess moisture, if you don't have access to desiccant packets.
tl;dr - Point is, the guy with water damage to his 7D likely ruined it himself by trying to keep using it after getting an error, that he acknowledges was caused by the rain... without drying it out first.