It'll be that meat gives you cancer or something along those lines. In truth, it's true that processed meat like bacon, salami and pepperoni are Type One carcinogens. However, red meat as a whole has been scientifically certified as a "probable" cause of cancer. No tests on humans have ever been conclusive.
Basically, if all you eat is red meat, you have an increased risk of getting certain types of cancer. It's not nailed on, of course and there are other health problems caused by eating too much red meat. But there are health benefits to eating lean red meat and no white meat has been scientifically proven to cause cancer either.
It's a cop out argument, as there are literally thousands of things we do that are avoidable that can give us cancer and even after that, we could do none of it and still catch the bastarding thing. Using a laptop or sitting too close to the TV can give you cancer. Drinking certain alcohols, smoking, working with fossil fuels, The Sun (not the paper, the star (also not the paper)), genetic abnormalities... the list is endless.
I wasn't teaching you to suck eggs by the way, IIRC you have some scientific background so you probably know all this, I just figured this would be the comeback he chose, as it is with most vegans.