What were you going to cook before you knew they were vegetarian?
You could always substitute the meat component for a shop bought veggie option and do the same salad/vegetables you were going to do. There are so many tasty veggie things in supermarkets these days.
If you wanted to do a spag bol or chilli, use quorn mince instead. Or buy some veggie burgers, there's usually loads of choice, and serve in a bun with salad and wedges. And buy in a nice pud, or a mix of fresh berries in meringue nests with cream.
The person is coming to enjoy your company, as long as you serve up something edible without meat I doubt they'd be bothered or expecting a gourmet meal.