I dont think blaming Kayleigh for whatever the torture of her own son is exactly productive.
First It is important to remember no one expects to suffer a very tragic and early death. When you appoint someone to stay with your child in case of your sudden death, it is not something you expect to happen. Maybe If she was sure she would die, she might have payed more attention on this specific decision and maybe even reconsider on telling Wymack.
It is also important to know people like Tetsuji can be EXTREMELY manipulative. He was the one to train Kevin and Riko to act in front of the cameras. Someone who watches the way Riko acts on TV would never think he was waterbording someone for funsies in his free time. It is actually very believable that Kayleigh didnt know. Tetsuji genuinely liked her (at least according to the EC) and probably made a real effort to keep her around and hide the darkest parts of his life and himself. He probably had what would be pretty close to a "media" personality near her.
Yes, she might have noticed signs or weird things. But those signs are gennerally only clear or obvious when seen in restrospect after already knowing the truth. Anyone who has already been a victim of someone who is both cruel and very manipulative knows this. When things are happening, you brush It off, you dont want to fight with someone who is dear to you because of a bad impression, you feel like you are exaggerating or seeing things. Then, you learn the truth and all of a sudden everything makes sense and all the little signs you brushed off make a clear painting.
No one lives their life expecting their best friend to be a Yakuza psycopath who will torture your child for pure sadism once he gets the chance.
She probably disagreed a lot with Tetsuji and thought his way of playing the game wasnt right, but Tetsuji never went around saying his methods included starvation and beating his players to a pulp. Yes, he talked about putting victory above everything, playing with precision and technique, giving your all and not caring about a certain level of violence on Court. Which are all things I am sure Kayleigh disagreed with, but no one is going to end a friendship of decades because their visions of the game are different. There is a mountain of difference between a more agressive playstyle and whatever the fuck Tetsuji was doing in the Nest. If Kayleigh only knew the first, you cannot expect her to deduce the latter.
Lastly, It is important to know that by the time Tetsuji built Evermore and started the Ravens, him and Kayleigh were living in different continents (at least according to the EC, so maybe Nora might change It, or maybe not). They created the sport togheter in Japan while in college, helped stabilish Exy there and then Tetsuji went to the US to start developing Exy there (which lead him to creating Evermore and the Ravens) and Kayleigh went back to Europe (where she was from) to spread Exy. This means they were mostly communicating through letters, phone calls and maybe emails later on (this was the 80s and 90s yall). They probably saw each other for real a few times a year. So It wouldnt be hard for Tetsuji to hide what he was doing with the Ravens and his nephew from her.
As much as It would be very understandable If Kevin or Wymack resented Kayleigh a little for her decisions, since they are emotionally involved in the situation, I dont think It is right to blame her for not expecting the horrors that would ensue her death.
Also worth to note that she never gave Kevin to Tetsuji. The whole reason why she was killed was because she refused to transfer Kevin's guardianship to Tetsuji, she wanted to be the one to raise Kevin. Yes, she made Tetsuji his godfather and accepted his help, but she always refused to let Kevin be fully raised in Evermore with Riko, as Tetsuj offered many times. This was not really what she wanted for Kevin. She wanted to be with her child. Even being a single mom with a very demanding work life, she still wanted him. Again, no healthy person expects to die young and suddenly. I dont think Kayleigh ever expected to leave Kevin alone.
This whole conversation is based a lot on speculations, but the information we have lead us to believe Kayleigh was indeed a very kind person and loving mother. This doesnt mean she is perfect and I do think she was wrong for hiding the truth from Wymack. But making a wrong decision doesnt make her complicit in abuse or means she is to blame for the horrible things Tetsuji did to her child.