Defamation actions are rarely successful in America. See RHINEHART v. SEATTLE TIMES, 98 Wn.2d 226, 654 P.2d 673 (1982). (mrsc.org)
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What is your legal question? You have not posed any questions. If you don't want to drop the complain, don't drop it. An employer is not a court of law. It is not bound by the rules of investigation and evidence or "innocent until proven guilty". Your employer can rely on flimsy allegations and hearsay to dismiss and discipline. Your boss is giving you his honest opinion that you'll do more harm than good for by filing the complaint. If you don't want to drop it, then don't. Keep in mind that WA is an "at will" employment state. This means you can be fired at any time without just cause as long as it is not related to your race, religion, ethnicity, age, gender, sexual orientation, disability, familial status, and not in retaliation for whistleblowing illegal conduct, taking FMLA leave, and filing for disability/workers compensation.
We do not have an attorney-client relationship. I am not your lawyer. The statements I have made do not constitute legal advice. Any statements I have made are based upon the very limited facts you have presented, and under the premise that you will consult with a local attorney. This is not an attempt to solicit business. This disclaimer is in addition to any disclaimers that this website has made. I am only licensed in California.