Sewa's right to be a bit shocked. The typical course of action is the girl going shopping with her friends to pick a swimsuit she'll wear to the beach with her boyfriend. Doing it the other way around is a serious infraction of the trope !
Also, I'm glad he at least tried to figure why a rich girl like her doesn't already have plenty of swimsuits (other than the mangaka wanting to draw a try-out montage).