First off-are you a MLP: FIM fan? Because if you are, she's my favorite pony
Secondly, animals can be surprisingly adept at squeezing through small spaces, fish included. I wouldn't be surprised if your girl managed to get through what we would see as an impossibly small space to try to get through to the other side. It's happened lots of us at some point
Unless someone's pranking you guys! Though I'd try to fortify the barrier more, cause they might hurt each other, and even breeders who have lots of experience with bettas know that there's always the risk that the male will turn on the female, even during the breeding process.