Protein is key to muscle building. Meat is a way to get that protein. You choose not to eat meat, but he does. That's that.
I did a quick search on best sources of protein for muscle building, and they are whey, eggs, meat, fish (first link). None of which are vegan. Another source had their top pics as dairy, eggs, and soy (2nd link) so there would be 1 vegan option, assuming you eat soy.
Bottom line is that most of the preferred protein sources for muscle building are animal products.
You weren't clear on what your argument is specifically, but I assume it's the need or importance of meat/meat products in a diet for someone to be strong and muscular. It's not mandatory, as you are proof of, but it's beneficial and many find it much easier to get the required amount of protein they need, when they include animal products.