(I was actually thinking of this earlier today. Maybe when they were journeying, they met up and decided to travel together. They did the gym challenge together when she was 10 (him 11 since they're a year apart). They were friendly rivals but became close friends. Either they part for a year or just continue to travel Hoenn for a year together. Then when she's 12, they both take up Coordinating. For whatever reason, they become extremely competitive and begin to have arguments which drives them apart. They end up pitted against each other in the Grand Festival (*prefers anime contests*) and he beats her, placing her runner up.

Maybe during that battle, in the heat of the moment, one of them said something that made the other angry, so they stopped talking for a while. It would have likely been her since she tends to trash talk her opponents in gym battles. Unless you'd rather it be him.

But when she became gym leader, out of professional respect, she started speaking to him again, though curtly.

During the past five years (since she became leader at 16), she has realized the error of her ways (or forgiven him if it was him that said it).