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! :
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