As a vegetarian she's going to have a ball in the UK, especially London. There is literally a world of veggie food available to her there.
I'd take her to a 'classic' like Cranks (is it still there?), which was probably about the same, when first established in the early 1960s, as French vegetarian restaurants are now - but which rapidly became more accomplished.
Then tour the world! Indian, Chinese, Thai, even a chain like Wagammamas is better than French vegetarian food - and the reliability of being able to go in and get something veggie, fresh and good is marvellous in comparison.
In French restaurants, even specialist veggie ones, you mostly get versions of classic vegetable side dishes. Plus fish! Do check whether she eats fish, as there seems ot be a widespread assumption in France that veggies do.
As for lunch at home, simple soup or quiche, with salad and cheese would be fine. What's good about everyday French food is quality ingredients that taste of what they are, not like processed food and supermarket veg here. Good fresh food is good fresh food.
The French myth of British food being terrible dates from the post-war period and focuses narrowly on traditional British food, ignoring the vast array of other cuisines that have become established favourites here - both because our tradidtional cuisine is so limited and, we're not so precious about it as the French are about theirs.