Assuming he was about to click "couple" and Frieren interrupted him to get her senior discount, she technically lost them a ¥300 discount for the pair.
It's actually worse than that. If we assume that they would have split the ¥3000 evenly had he picked "couple", her actions mean she forces him to pay ¥500 more (¥1500 -> ¥2000) so she can save ¥200 heself (¥1500 -> ¥1300).
