All the grass growing in the borders or in the lawn?

No plant will kill off grass - but some plants, sown at the right time in a wildflower 'meadow', will out compete any grass that tries to grow as time goes on, maybe that's what your daughter meant.

I remember dealing with a question about that plant, but now I can't find the question or my comments on it.

Aha! I found it! The plant is Yellow Rattle (Rhinanthus minor). It is a root parasite on meadow grasses, slowing their growth, and giving the flowers a fighting chance.

Mowing the field twice a year helps to control Couch Grass growth. Before and after the Wild Flowers have bloomed.

The National Wild Flower Centre at Alderhays Liverpool will send advice to help you. They are very good.

It was Monty on Gardeners' World. I made a note of the Yellow Rattle being useful for weakening the growth of grass in wild flower gardens.

Dianebulley, is that before tall growth in spring, and after seed ripening for the flowers? Mowing after flowering stems form, and before the seeds ripen, will weaken the flowers.