wanting to say something as well even though most have already made such fabulous comments.
I'm a vegetarian for 25 years now.
While at the beginning I would cook meat for my family when visiting, I stopped that a long time ago as now there a so many great alternatives that nobody really minds anymore eating my vegetarian meals.
When I go visit others, I don't expect them to make a meat free dish or an alternative for me.
But I do expect that there is something that I can eat.
I am fine with just tatties and some veg or salad.
But it would be difficult if you made a stew as there would be nothing not coated in meat juices.
If you don't want to make an effort, just heat up a veggie quiche for your friend, or even serve it cold, or have a veggie ready microwave meal or somthing. You don't really have to "work" to provide that.
What confused me a bit was, that you stated that you used to be vegetarian but would eat meat when it was served to you when visiting others. Then you were not a vegetarian, you just choose to eat no meat when you cooked for yourself - great difference. But surely that would mean that you know how to cook vegetarian and would have more sympathy for your friend.
Just to add to everyones knowledge a few facts
If someone follows a vegetarian/vegan diet, their body is no longer used to some animal fat/proteins. If they would eat it, they would become very sick.
Be aware if you cook for a vegetarian that you do not accidently use animal products often hidden in ingrediances just as lard (on roast potatoes for example) or animal stock cubes (often used for soup) or gravy granules.
As a vegetarian I really don't want to be fussed over when eating at a friends, it's really embarrasing. I don't mind if someone does not want to cook vegetarian but let me know beforehand, never had an issue to bring my own meal with me.