There's no clearing customs or immigration when leaving the U.S. (That's why international departures don't have to use the Tom Bradley International Terminal, though those of airlines that fly only internationally from LAX, such as Qantas, do anyhow if they have no gates anywhere else.) The check-in agent at SEA will make sure his passport is in order. This will probably be repeated once or twice more, since UA doesn't want to have to fly him back at its own expense if Australia won't let him in. That doesn't add more than a few seconds to the process, since the people who will look at his passport would generally have to see some sort of ID anyhow.

As for security: he'll clear it in SEA and will stay airside at LAX so he won't have to clear it again there. That's a real time-saver.