I taught ESL for a couple of years to Korean kids. The best way to do it is unstructured talk sessions after you've got to know them a bit. Since this person is already your friend, that makes it a lot easier.

Pick a topic each time -- something very broad such as "buying something" or "colours" and go from there. If you get off topic, don't worry about it. The idea is to get them talking and interacting in English, not concentrating on memorising certain vocabulary. They can do that too, but conversation will help them a lot more. Get that right and the vocab will happen almost automatically.

You need a lot of patience and you need to develop a certain skill in simplifying the language you use, and how to explain things. I'm assuming from your post that you speak this girl's native language: don't use it in lessons unless you're REALLY stuck for an explanation. Draw pictures, act things out, be creative. I went through a metric ton of sketch paper with my students.

As another poster said, children's books are great. Find a book that's at the level she can read in English, have her read it to you, and go over anything she doesn't understand as you go. Once she understands the book, have her put it down and tell you the story in her own words.

Virtually all of teaching ESL is about getting them over their lack of confidence so they can open up and talk to English speakers without fear of being word-perfect. Once that happens, they'll start learning for themselves at an unbelievable rate. At a beginner or intermediate level, don't worry about correct grammar. The purpose is to get them communicating, and that means "getting across what you want to say so people can understand it", whether or not it's great English.