I think the answer to the OP's question lies just as much in where the friend wants to be. If, for any of you, this is likely to be the only, or one of the only visits in a lifetime to the Bay Area, go to SF. It's a world class city with a world class setting. However, avoid Fisherman's Wharf which is a very nasty tourist trap development.

If, however, you all are fully familiar with San Francisco and want to explore somewhere new, then by all means stay in Oakland. It's rough but it has a nascent food scene (fed by cheap property) and a handful of mildly interesting places (although you should be aware that a large proportion of Bay Area residents won't go near the place - it holds no interest to them and it's dangerous). Whatever you do, avoid the many slum areas, don't go anywhere near the increasingly violent Occupy protests and don't wander around at night.

Alternatively, if you want to stay in the East Bay, Berkeley might be a compromise. It has a good food scene and the University provides enough cultural interest for a weekend.