What size is the tank? Eventually, there is going to be some beating up. There's just no point to keeping them together, they WILL fight, even if you think they "get along" bettas do not need betta buddies. It's a selfish thing to just want them together because you think they get along. At some point, they will tear each other apart. I would get the girl her own tank, it's worth it, then you wont have to deal with healing both her and his torn fins someday. Or fry, for that matter.