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To me, whistle blower has a positive connotation. The "whistle blower" here was a partisan hack that LIED about the contents of the call he didn't even hear. Other options are:

Brutus

Judas

backstabber

betrayer

defector

deserter

double agent

double-crosser

fink

informer

quisling

snake in the grass

snitch

spy

stool pigeon

treasonist

turncoat
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