I started diving in Korea, and have since moved to California and am now diving Monterey as often as I can.
As someone said above, Korea is not at all a diving destination.
However, if one wants to get wet, there is some local temperate water diving not too far off.
From Seoul, the East coast is maybe two and a half or three hours away, good for a day trip, and the visibility is around 20~50ft. Places such as Kangnung, Sokcho, Donghae, etc. all have a number of dive shops that rent equipment and take you out on boat dives.(mostly inflatables)
You need to wear 5mm farmer john/jackets.
There's not that much to see, but it's still the ocean, with some fish and invertebrates, so beats any quarry dive.
Good luck finding an operator who speaks adequate English, though.
The west coast is not worth diving, too silty.
The south coast is OK but too far away.
Cheju(Jeju) Island is pretty nice. It's an hour from Seoul by plane, and Sogwipo, where most of the diving takes place, is another hour by bus (there is a shuttle from the airport to Sogwipo). The water is subtropical, ranging from 65~75 degrees from Winter to Summer. The fish life is pretty interesting, and the offshore volcanic islets have 200ft walls covered with soft coral. Visibility is usually around 40~50ft.
The best bet here is Bigblue33, as someone already pointed out. I've never dived with them, but the owner is a German who speaks English, so it should work out pretty well.
There are many things on land that are worth checking out, too, BTW.
All in all, if he wants great diving, go somewhere else. (SE Asia, Guam, Okinawa, etc.) If he just wants to check out local diving, try the East coast for day trips to get wet, and try a weekend at Jeju island for a more interesting experience.