A friend of mine grew up speaking basic English at home (living in a foreign country), but it was not more than that: basic. There are many words that he hears people say that he knows are not among the most common words, but are common enough that if he were to say them, they would be understood. Some examples that came to mind: consensus, contribute, distribute, perimeter, etc. I suggested to him to look at dictionary.com's word-of-the-day, but we looked through a few, and they were quite florid words (one of my favorite autologs). I do not think they would have "consensus" on that list. What is a resource that he can study from that will help him build his vocabulary beyond a basic-level English? The larger the word-bank the better.
Just read. Read a lot. Read about a wide variety of topics. Thinking about what qualifies here, I'd start with periodicals. Newspapers and magazines are released regularly and often have diverse topics to cover. If you're looking for a completely free answer, try a news site online. NPR and AP are both factual sources. The more you read, the more likely you are to come across a word you haven't seen before.
Also, there's nothing wrong with just looking through the dictionary.