Best Porterhouse in SF
DavidT's caveat is an important one - your friend should realize that this is not a great steakhouse city.
That said, Harris' will serve you a fine dry-aged porterhouse of decent size.
Acme is your friend's best bet if he likes his porterhouse to be cut from a grass-fed side of beef.
Alfred's on the other hand, is the best bet if your friend likes his porterhouses to be very large. Their normal 'King of Steaks' porterhouse is 30oz. and no mean feat to finish. Not on the menu, but still available upon request, is their 60oz. 'Kingdom' porterhouse. Easily two-inches thick, it takes a mighty appetite to consume all that beefy real estate.
One of my personal favorite porterhouse in the Bay Area can be found a short drive outside of SF in Walnut Creek. At Vic Stewart's, you can dine upon their sumptuous 49oz. Angus porterhouse steak. Finish it [and the accompanying side dishes] and you are a member of the '49er Club' with your name engraved on a plaque on the wall and a certificate of membership mailed to your home several weeks later.