So, based on what Fantasy and I have talked about with solider Riley, basically...
Riley gets kidnapped by the Japanese and is implanted with a chip that turns her into "soldier Riley" by basically programing her to kill other Animorphers. So she has no idea what she's doing and when they are able to remove the chip, she doesn't remember anything that happened while she was chipped, including Cole's death.
I'm not sure if this has been mentioned on the DT yet, so if you're interested in what Fantasy and I were tentatively thinking for Cole's death, I have enspoiled it.
Spoiler
The Japanese chip Riley, turning her into soldier Riley and lets her out in the world to track down the other Animorphers. She finds Cole first and he figures out pretty fast that she's not herself, so he calls the other Animorphers to warn them. Maybe by this point, they know about the rebels (and Darrin) and turn to them for help. Maybe the rebels have hackers that try hacking her chip, but all it does is make it cut in and out. So, with Cole trying to escape Riley and buy the rebels time, he ends up on the rooftop of a building and Soldier Riley pushes him off, causing him to dangle from the edge. Then her chip cuts out and she tries to save him, but it's too late, and he falls. She makes it to him at the bottom before he dies and is able to say goodbye, then her chip cuts back in and she leaves. Then the Animorphers would have to come up with a plan to capture her and, probably with the help of the rebels, would unchip her.
