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The person who is told off is "the whipping boy".
A:"Did you tell the boss off?"
B:"No, but John was with the boss when the boss made the mistake, so I told John off when he was with the boss."
A:"Ah! So you used John as the boss's whipping boy."
B:"Exactly - I think the boss got the message."
OED:
whipping-...
1
hidden agenda : an ulterior motive m-w
I think the headword and its definition here could both work in your example, in fact:
ulterior motive: a secret reason m-w
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