Meat indeed refers to the flesh of another animal, but it isn't always attributed to animals.
It can also refer to plants, though to use meat in that context, you would need to be in that context.
The meatiness of the broccoli was very tough.
The meat of a tree has two parts, the heart and the white part.
Meat is defined as the most consumable part of something, rather it be an animal or a book. Although it isn't very used, one could say something like:
The meat of the story is in the beginning of that book.
It can refer to the chief part of something.
The meat of the music is the bass.
Though are very nontraditional usages of the word, but they are understandable. The usage of the word
meat in those sentences add emphasis, because
meat is attributed to the good part of something.
I will agree with you, though, on your argument with your friend. Meat is not exclusive to mutton. The only time it is exclusive is when you already know what the
Suppose you are eating lamb stew. You could easily get away with just saying,
This meat is tough.
It would indeed refer to the lamb, assuming that was the only meat in the stew.
To even further prove your correctness with your friend.. You can order meat-lovers pizzas and traditionally, mutton is not on a pizza.