Take a train instead of a flight. Cycle to work or take public transport instead of driving. Install a heat pump or solar in your house. There are a million things people can do to cut down their emissions that can be as effective as becoming herbivores, depending on each one's personal situation.

Plus, I don't have the numbers in my head but I'm pretty sure a locally grown fillet of chicken is more environmentally friendly than an avocado that has travelled across the Atlantic, so "buy local" would be probably better advice.