astronomy - How can a full moon be seen south of an observer's location?

I know this seems like a simple question, but I'm trying to debate with a flat earth theorist. I asked him to explain why can the ISS visibly be seen orbiting the Earth with the naked eye, and he put this question to me instead.

He asked:

"How can a full moon be observed south of an observer's location, despite the fact that if the moon is illuminated by the sun, an observer has to be almost directly between the sun and the moon to observe a full moon?"