Boy! I am probably the last person to answer this one....our family is such a group of carnivores!! I would say, however, that reinforcing any healthy choice made by a teen is a smart move!
Be sure to add a good variety of vegetables to the fridge so that your teen has a lot to choose from when it is meal time. I might also sit down with my teen and look at meat alternatives, and our budget and see what ideas we could put to work in our meals. It might also be a good time for your teen to begin assisting with some cooking. If he/she wanted to narrow their food choices to only fruits and vegetables, they might be interested in working up the dishes that are meat free.
Encouraging them to take an active role will do one of two things: make them change their mind or reinforce their good decision.