A larger project—like a life story—is certainly an exciting project. You've already identified something intriguing about the story. Finding the right shape for that story will be your next challenge. And a class in the longer form can be very helpful with that. Let's back up a bit, though. First, you’ll want to decide how you’re going to approach this story. This decision isn’t always clear-cut, particularly when you’re telling someone else's story.
One option is to write a novel, using his life story as inspiration, but giving yourself room to fictionalize. Dave Eggers recently did this in his book What is the What, a fictionalized account based closely on actual events experienced by Valentino Achak Deng, a "Lost Boy of Sudan" who fled his home because of war and later immigrated to the United States. Eggers interviewed Deng at great length, but ultimately fictionalized the story. He felt this would allow him to bring in a stronger level of detail to events he did not experience himself.
Another option is to write a narrative non-fiction. (Technically, you wouldn't be writing a memoir because this isn't your story. What you're writing might be better termed a narrative non-fiction, meaning it unfolds like a story—as memoirs do—without the implication that it's your own story.) In The Zookeeper’s Wife, Diane Ackerman tells the story of Jan Zabinski, the director of the Warsaw Zoo, and his wife Antonia, who helped shelter hundreds of Jews during World War II using the city zoo as a refuge and hideout. Ackerman drew from Antonia’s diaries, survivors' stories, wartime photographs, and film footage in order to create the narrative.
If you feel like you have enough detail—or can get it from your friend—and want to stay true to his exact story and impressions, you might work within the confines of non-fiction. If you’d like a little more freedom to imagine your way into these experiences, you might fictionalize. Neither option is inherently better. It comes down to what you want the book to accomplish, the nature of the resources you have available, and your confidence in using those resources to create a vivid narrative.
There's quite a bit of overlap in the craft of writing narrative non-fiction and fiction. Both demand, for example, fully fleshed out characterizations and a strong setting to ground the reader in a specific time and place. In many ways narrative non-fiction unfolds as fiction does, with the scene as the basic unit. So both a fiction class and a memoir class would be beneficial. However, there are elements that are unique to each genre and it's important to have an understanding of those going into a project like this. At some point, you'll want to take a class in the genre you choose for the book.
If you have a strong leaning toward one option over the other, go in that direction as you choose a class. Although this technically isn't a memoir, a class on the topic will still help you with the craft and it will give you an opportunity to explore some of the issues that arise when committing truth to the page. This is important to all works of narrative non-fiction. If you're not sure what approach to take, discussing these issues with other memoir writers might even give you clarity on how well you would be able to stick to the truth and still dramatize this life story.
Also, you can discuss which approach would be most beneficial with your instructor and fellow students. A writing class is a community and they may be one of your more valuable resources in making this decision.