Does he like to bike? If he can rent a bike during the time he is there, I can't think of a more interesting and cost effective way to get around the city. My favorite ride is to bike across the Golden gate bridge and then up a *really* steep hill on the other side to a lookout point over the city.
The muni/bus system is pretty slow but reasonably priced. $2 for a ride anywhere in the city and you can transfer for up to 2 hours.
The farmer's markets are all very nice (I like the one at the Ferry building on Saturdays) but they aren't budget.
You can't go wrong with a burrito in the Mission area. Also, I can highly recommend Cha-Ya, a vegan Japanese restaurant which is very reasonably priced. There are tons of other great restaurants on Valencia.
On the more touristy side, he should take a cable car ride through the city and stop in to Ghirardelli Square and get a sundae. Golden Gate park is well worth a visit too and I like driving down Lombard St (crookedest street).
I can give you more info if he enjoys any of the following: museums, wine, biking, hiking/walking/the outdoors, food, improv. Also will he be there during the week or will he be there over a weekend?
