To be more exhaustive regarding Diane, I could even say :
- Haley+Sandra (story over, minus epilogue): whatever choice, story with Diane is over, for a long time; Diane only appears in the end to say how she envies the MC for having found true love.
- Haley+Adriana (story not over): MC and Haley have moved out; the main reason Diane was around was she was their neighbor, so...
- Haley+Lisa (story over, minus epilogue): Diane has not appeared at all in this route since it started, and the story is basically over. Given how insecure Haley is in this route, I don't see Diane making a come-back.
- Haley only (story not over): that would be the only route with the story not over where Diane could physically come back (they are still neighbors, offcicially the MC is not engaged to anyone, etc.). I don't remember exactly her last appearance in the non-Sandra routes, but I think it was a while ago (probably the seance to the theater with the girly movie?). And, on the specific Haley only route, the atmosphere has drastically changed and feels more and more like a "MC+Haley vs the rest of the universe" thing, with increasing tension/dramatic tones. I don't see a perky Diane fit in there.