A wide shot reveals that Toby is in a king-sized bed all by himself and the other side does not look slept in, so... the show seems to be indicating that the "her" in question is actually Kate. But I don't think so. I think it's Rebecca, who would be in her late 80s at this point. I think she's dying, and they're all gathering at her bedside. Now, Kate could also be dead here. I would not put it past This Is Us to have a shocking reveal that Rebecca is dying followed by an even more shocking reveal that Kate is already dead. But she could also already be at her mother's side, and that's why Toby is alone.