I'm questioning my friends food preferences after seeing what meat they ate
Why are you questioning his food preferences?
That's totally insane. I think vegetables are always more healthy than meat
Friends are expected to support one another. If your friend has his own food preferences then you shouldn't question them. They might feel like you're being judgmental if you do.
I think my friend is an idiot. He knows that eating meat is not good for his body.
It's still his his body, and I think it would be wrong to try and police what he can and cannot eat. You can try to persuade him for the sake of his health, but ultimately it would be best to just leave him be and make is own choices.