To me, this sounds like a guy and a girl secretly loved each other in the past but they never admitted it. The guy got over it, and both of them found new loves and moved on. Now, however, the girl is back in the guy's life, and he's remembering what he really liked about her in the past, but he knows it's too late for them to be together, although she doesn't want to accept that. She's his "day late friend" because they can't be together.
To me, this sounds like a guy and a girl secretly loved each other in the past but they never admitted it. The guy got over it, and both of them found new loves and moved on. Now, however, the girl is back in the guy's life, and he's remembering what he really liked about her in the past, but he knows it's too late for them to be together, although she doesn't want to accept that. She's his "day late friend" because they can't be together.