Sexuality can fluctuate. I have known bi people who at some point in their lives only felt up to dating a specific gender or the other.
If she has described herself has a lesbian recently, it's possible that right now she's only feeling attracted to women. However, it's also possible that, having been attracted to males in the past, she feels that she might again later, so she's giving herself time.
It's also possible that she was gay all along. I'm just adding a new possibility here.
Either way, there isn't much D can do but wait and not expect anything. If she wants to go back to him, she will, and if he wants that too, great.