Get the girl and keep the girl
If you can flirt with the girl to get her then you can flirt with her to keep her. So many of us are so quick to place blame as to why a relationship fades. You have to stay interested in one another, genuinely. Instead of getting upset over the things they do or don't do that upset you, think about the things they do that contribute and make you feel good. It takes effortless work. He should give her time and she should be thankful for it. Embrace each other for what you actually do rather than what each other doesn't do. Even if it's something so small, it's important to acknowledge it.