I dated the same boy all through high school and through our first year in college, from the age of 15 through 19, and he was my best friend also. We ended up going to different colleges and saw each other about every other weekend then. But by the end of Freshman year, we discovered we were growing in different directions and that we should also date other people to know what it was like. We still loved each other, but knew something had changed.
After one awkward year, we were able to keep our friendship intact which lasted. When I called him to tell him I had gotten engaged, turns out he had gotten engaged the very same night. We remained friends up until his untimely death at age 36 from a brain tumor. I miss him still and remain close to his mother.
Point is, whether they stay together or not as a couple, he could still be a positive fixture in her life for a long, long time.