Any suggestions for the best restaurants in SF that are on the cheaper side (lunch and dinner)

I have a friend who is visiting the city for a few days, and unfortunately I’m not going to be there to show him around. He’s from Uruguay, but speaks fluent (or almost fluent) English. He’s also never been to California before.

I believe he wants to eat well, and diverse, while he’s there for a few days, but he’s on a bit of a tight budget, so restaurants, cafes, and sandwich shops on the cheaper side would be preferred.