@crispix:
There's been a great deal of quibbling on this thread. I'll try to keep my comments on the constructive side. The gas planning thing has been discussed at great length.
I'm familiar with the La Jolla Shores area. In fact, I do most of my local diving there. It's a wonderful, safe place for diving...even for beginners. I would say that it's even more appropriate for beginners than a shallower (25 - 30 fsw) La Jolla Cove dive, which can have significant surf, bad visibility, kelp "entanglements," and sharp rocks presenting navigational challenges.
One aspect of the dive profile that many on this thread don't know about is that a La Jolla Shores dive is a
shore dive. This means that there isn't a boat that you have to locate once you surface. A team can surface at any point during a dive and still see the shore nearby. With an easy surface swim and typically no significant current, the dive team can make it to shore without difficulty.
I'm assuming that the dive in question was to the Main Wall (just west of Vallecitos). The edge of the wall there is located at 50 - 55 fsw. In some places the wall is as high as 7 - 8 ft. Often times this is used for the "canyon tour" at the end of OW certification dives. I mention this because if the beginner diver was certified in San Diego, then there's a very good chance that he was familiar with the dive site. In my opinion, this goes a long way in making the beginner diver feel more comfortable.
The topography from the Main Wall to the shore is very gently sloping sand -- there are no significant drop-offs. Often times, divers swim in along the bottom and end the dive in waist-deep water. There can be some surf to contend with, but most days it's only 1-2 ft waves. I'm posting this info so that others won't think it's reckless that you decided to take a beginner to this dive site.
For future dive planning with a beginner diver, I would head out directly west from Vallecitos. Using the proper line-ups, you can drop in about 20 feet from the edge of the canyon in 30 fsw water. This minimizes the amount of work they have to do to get to the canyon. Head north along the wall towards the Main Point...periodically checking the SPG of the beginner diver. Proceed slowly...and point out some of the life on the wall. The beginner will likely hit the turnaround pressure before making it to the Main Point. Even though I'm checking his gauge quite frequently, I generally tell the beginner diver to let me know when he hits 1500 - 2000 psi (if he's using an AL80 tank). Take a 90 - 110 degree heading back to shore. Emphasize to the beginner diver that it isn't a race to get back to shore. Tell him to take his time. There's lots to see in the shallows: guitarfish, sting rays, bat rays, leopard sharks, pipefish, and crabs. The safety stop is generally done while swimming in very slowly along the bottom. Often times you can find a convenient place to check out a pipefish at about 15 fsw.
Most times the beginner will hit 500 - 600 psi well after the safety stop has been completed. If that's the case, then simply give the "thumbs up" sign and end the dive at the appropriate time.
There's one more thing that I'd like to mention. I can totally understand why you and your buddy had HP100s whereas the beginner diver only had an AL80. For one, I don't think it's possible to rent a HP100 steel anywhere in the San Diego area. Since the beginner diver is renting some if not all of his gear, chances are that he's using an AL80. I could also understand why you wouldn't want to switch tanks with him. Doing so would give him a heavier tank and change the distribution of weight on his rig. If he's never used a HP100 before, he might have more problems managing the weight along his back. I know that some beginner divers complain that they have a tendency to turtle when using a HP100. Switching tanks would also require you to adjust your weighting (but I suspect that would be a trivial adjustment for you).
If you're still out there monitoring this thread, I hope that you found some of the info in this post useful. I also hope that you get another opportunity to dive with the newbie. It can be very rewarding to witness the growth/learning of a beginner diver...and know that you played a small part in it.
Have fun and dive safe...