Your friend is mistaken. Most tomatoes are self-pollinating, meaning that the pollen from another plant isn't even likely to reach the flowers. Even if stray pollen were to make it to a flower, it would have no more effect than dust falling on the plant. Plants must be closely related for pollen from one to have any effect on the other; even then, the results wouldn't affect the fruit, only the next generation's seedlings. Edible flowers as well as herbs are natural partners for tomatoes. Even sun-loving ornamental flowers, though, will add interest and color.