It's not a stupid question, so don't feel bad for asking it. In fact, it's quite a discussion and many people in the community disagree. I guess the three of us just stepped forward most quickly and happened to be of the same opinion. As I said, the other side, which would say as your friend did, does have some valid reasoning.
I think it's more just the way your friend framed it, like "you can't enjoy it unless you understand everything about it." On one hand, that makes sense, but on the other, there are many things that people sometimes don't understand that they enjoy. For example, I think lightning storms are absolutely beautiful, and I understand the basics behind how they happen, but I don't understand the intricacies of the meteorological situations and such. If I did, it might deepen my appreciation for the phenomenon, just as knowing Japanese might help you appreciate the show/manga more than you did before, but things like art, storytelling, character, and even voice acting, regardless if you're a sub or dub person, can all be appreciated.
Another example is that I often listen to music in various languages, including my mother tongue of English, but also Spanish, German, Japanese, Korean, and Russian--languages which I may only know very little of (despite having been studying Spanish for about four years), and yet I can appreciate the skill of a singer despite their language not being mine. I'm also a person who likes to appreciate a culture through its own lens, and again, doing that helps me understand and enjoy more.
It's a very deep and well-meaning question. What does it mean to enjoy, and what does it mean to understand? How are they related? Does one require the other?