Having someone send you messages about their day is awesome. And when there's so many texts of that nature, there's normally no obligation to respond. He should've made that clear to the sender. Senpai should've also limited it to a number above one per day if he really wanted to set a limit.
This girl might be too pure to turn yandere. I think he should go for it and slowly condition acceptable behavior, just like with the texts. Although, I think that could've been done much better, as well.