I had a big rooftop garden recently, with everything in containers. My number one choice would be passionfruit, if the person is willing to put up something for it to grow on. Looks great, easy to grow in San Diego, drops fruit when they're mature (takes the guessing out of it), and does well in a large container (15 gallon or larger). Frederick is easy to find at nurseries and is very vigorous, so it's a good option for someone new to it.
Figs also do well in large pots, but they require quite frequent watering or they wilt. (This is the opposite of figs in the ground -- in-ground figs are quite drought tolerant. I think the difference is that a fig in the ground can send its roots far and wide in search of water.)
Pineapple is also a nice one, and easy to grow and fruit in a 15 gallon pot, and requires minimal watering and care.
Pineapple guavas will also do well, but like figs they're much more sensitive in a container than in the ground. (They are amazingly resilient in the ground.)
Tree tomato is a nice oddity to keep in a large container -- it'll grow 15 feet tall with giant leaves in a 15 gallon pot and fruit heavily.