It's fun to do Spring Break in a place that's warm and sunny. There are a lot of fantastic things to do here: hanging out at Pacific Beach/Mission Beach, catamaran sailing on Mission Bay, Temecula wine-tasting, the Wild Animal Park, the San Diego Zoo, Sea World, bar-hopping in the Gaslamp, fish tacos, and more sushi restaurants than you can shake a stick at.
Oh, yeah. There's also snorkeling and diving in La Jolla.
As others have mentioned, the water is cold (mid to high 50s at depth with no thermocline). 7 mm wetsuits are required. The rocky, sandy wall at La Jolla Shores is not colorful at all...but there is a lot of life if you know where to look. Open Water students generally don't see much on their check-out dives due to the short bottom times and the fact that they're concentrating on operating their gear. After certification, La Jolla Shores (certification site) would be the place to do a couple more dives. If you join the Divebums.com email list-serv, you can ask whether any locals would be willing to give a couple of newbie divers a tour of the canyon.
If your friend is willing to look at it as a vacation in San Diego with getting scuba certified on the side, then I think he and his girlfriend would have a great time. Getting certified in cold water isn't as difficult as people say it is. Once he's certified, he can consider making a trip out to Catalina. I can highly recommend going diving in Catalina, but I don't think that spending an entire week on the island would be as fun as hanging out in SD.
With respect to dive instruction,
Ocean Enterprises is currently running a 50% off special on OW certification. As others have mentioned, students are expected to buy their own mask, snorkel, gloves, booties, and fins. Your friend could purchase these things ahead of time and save a few bucks. FYI, I believe that OE gives a nominal discount for enrolled students. Another dive shop that does scuba instruction is Sport Chalet (two locations: UTC Shopping Mall and Point Loma; PL location has a swimming tank on site for confined water training). Sport Chalet is probably the best place to purchase any dive gear since the store has the lowest prices on everything.
One other thing to keep in mind is that a rental car is mandatory. Plan for it. Public transportation is just not that convenient here.
Hope this helps your friend make up his mind about his Spring Break destination...