El Gordo is possibly the most amazing taqueria I have ever been to. It's completely frantic at lunchtime, so know the system: there's one line for the adobada (spicy pork) tacos, one for the other kinds of taco, and a third for everything else on the menu. You're going to want plenty of the divine adobada, full of meat fresh off the spit and garnished with avocado, cilantro, onions, and just a bit of tangy sauce. But you also can't miss the flavorful lengua or the tender cabeza, so your friend should stand in the other line to order those. A third person, if you have one, should grab some limes and radishes and nab one of the picnic tables outside. A happy taco feast will follow.
El Gordo is also a late night lifesaver. I think there's like 5 hours in the day when they're not open.