A few years ago I went to a restaurant in France with a vegetarian friend. After looking down a menu with few possibilities, we eventually decided that the Onion soup would be fine. That decision turned out to be a particularly bad one when she unearthed a large bone at the bottom of the dish. I had quite forgotten that a decent onion soup would be made with a beef stock!
I learned my lesson that day, and now when I have vegetarian visitors, I make sure we go to restaurants that cater specifically for them. Finding these establishments is easier said than done though, which is why I was particularly happy to come across this exhaustive list of vegetarian restaurants in Paris on the very useful Posted in Paris website.
One or two of these are of the fairly hardcore variety and quite joyless places to eat, but I can particularly recommend Le Potager du Marais. Business must be good there as they recently extended in neighbouring premises!