You asked what you can do... I'm an attorney and so I can only give you an idea of your legal options. Colorado is an "at will" employment state. At-will employment means that without a contract, you have no contractual or other right to employment with the company. The company is entitled to fire you for any reason: a good reason, a poor reason, or no reason at all--as long as the company does not fire you for an illegal reason (race, gender, age, religion, etc...). But it extends beyond firing, to hiring, promotions, demotions, wage cuts and raises, disciplinary actions, and even scheduling. Unless you can show that this was done in violation of a contract, union agreement, or a clear violation of an unambiguous and binding clause against the employer, or that it was done because of some minority status (age, race, gender, religion, disability) that you have, then they do have this discretion.
So someone CAN be best friends with the manager, the manager can give the friend the best access to everything and restrict yours, etc... It's unethical, immoral, illogical, but still legal. The only way that it'd be illegal is if this was due to your race, age, religion, gender or disability.
Now as to any false statements of material fact to third parties that cause you quantifiable economic damage, you could have a case against the person that spread those false statements. This is called "defamation". But you'd have the burden of proof to prove those statements, prove the damage incurred, etc... and to be honest, it's not easy to prove these things, but if you have the evidence necessary to do so, it's still possible.
In short, you almost certainly wouldn't have a case against your employer, and most likely wouldn't have a case against the co-worker unless you could prove the elements of defamation. I agree that it's unethical, immoral and illogical, but not illegal.
I know this is probably not what you wanted to hear, but it is the law. I hope that clears things up anyway. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable.
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Thank you, ***** ***** luck to you!