Hall of Fame
Read the rule.Erm, what? Her coach admitted he was coaching right after the Osaka-match. Whether she saw it or not or wanted coaching or not, has no bearing on the warning. She's "responsible" for his actions.
"Players shall not receive coaching during a match (including warm up) with the exception of the allowed coaching breaks as defined in the On-Court Coaching Requests - Section XVII.H.3. Communication of any kind, audible or visible, between a Player and a coach may be construed as coaching"
As a former coach, it was on me to know these rules. Serena wasn't active in any communication. Don't be stubborn.