It is completely false. I do not eat meat at all and haven't for a long time. I recently went to the dentist and they showed my my x-rays. I have minor cavities between all my teeth from not flossing. Apparently, flossing does not just remove chunks of food, but also the harmful bacteria plaque. Between teeth is a great place to for plaque to hide because it is warm and dark and out of harms way. If you brush and do not floss, you miss all of that plaque so it just sits between your teeth and rots them.
You're friend needs a new dentist for sure.