❓How do I gently let a friend know I'm vegetarian before they treat me to dinner?

The earlier you mention it, the easier it goes. Once the time and place are settled, just casually slip it in, something like, "Oh by the way, I'm vegetarian now — if it's easy, maybe consider somewhere with more vegetarian options, but if not, it's totally fine, I'll figure something out." Keep the tone light and make clear you don't want to be a hassle — most friends will naturally take that into account without feeling burdened by it. If the restaurant they pick really doesn't have many vegetarian options, you can also suggest checking nearby for a place with more choices, or just mention you'll eat something beforehand. Leaving the choice and the easygoing tone with your friend usually means the relationship won't get awkward over something this small.