Alright, let's break this down one at a time.

"What is she doing in Hawaii?"

Shujin school trip, all of the student body was present. Only contrived part of that is Yusuke and Hifumi being present since they're Kosei students who were planning to go to an entirely different location.

"At the exact same time as Joker"

They're about to leave Hawaii, last-minute souvenir shopping is hardly uncommon.

"In that souvenir shop"

Again, about to leave. They aren't gonna be roaming around looking for souvenir shops, it's likely they just visited the one in the hotel.

I'll admit it is slightly contrived, but it's far from unreasonable.

Not even gonna get into my problems with that last bit.