River, sometime after the Library.
He's never going to visit.
She may not be corporal any more, but she's still living. The soul of a Timelord (or part Timelord anyway) is not contained within the physical. That stubborn, annoying, wonderful man understands, but he still blames himself for her loss. It's infuriating, that he can forgive her all that she does wrong, and yet never forgive himself. He's good at pretending, though
She knew he would hold onto the guilt of her sort of death. But she loves him too much to give him up (not counting the plethora of fixed points within their relationship and the potential paradox of preventing her existence in the library in the first place).
Years of a backward relationship, a stint in prison for something she didn't do, and a childhood with parents who didn't even recognize her, she's learned acceptance and patience. In her short (by comparison) lifetime, she's learned to deal with loss and loneliness far better than the centuries old alien she loves.
So she dwells within the database of a computer program and waits.