eat all their vegan food (but not without permission, that could be dangerous) (it's best to ask them to fix you a vegan meal, then force them to watch you demolish their masterpiece mouthful by mouthful, and enjoying it. Ugh.)

buy vegan food from the store, then the store will run out quicker and they'll be upset

become vegan, then when you inevitably shrivel up from lack of protein and your clothes fall apart because there's no leather and wool to hold them together, your vegan friend will see the futility of their ways

fix them a delicious vegan meal, they'll know how awesome you are at cooking and then recoil in horror when they realize what you could do with meat

don't