Everyone is allowed to have their own dietary preferences, whether you like them or not. They’re allowed to eat what they want to, and that includes when they’re at your house. You don’t have to accommodate them, but… it would be nice to. If you care about them and want to have them in your life, then you should make some sort of effort to include them in your dinner plans. Especially when it’s as simple as being a vegetarian. That is a pretty cut-and-dry dietary restriction. Cultures have been acknowledging it for centuries, so if you don’t get it, that’s on you. 

It’s one thing to ignore someone’s dietary restrictions. It’s a whole other thing to force them to eat something they don’t want to against their will. You wouldn’t sneak peanut butter into the meal of someone with a peanut allergy unless you were trying to seriously harm them. So why would you sneak meat into a vegetarian’s meal? Just to prove a point? I don’t think that would end up having the effect you’d want it to.