Employers in CO and at least 43 US states cannot lawfully fire an employee for reporting a crime to the police, or calling 911. It falls under the public policy exception to at-will employment.
Some examples or this are covered here. https://www.bls.gov/opub/mlr/2001/01/art1full.pdf
Two examples of statutory text, but most will fall under case law.
California Labor Code section 1102.5(b), an employer is prohibited from retaliating against an employee who discloses information to a law enforcement agency where the employee has a reasonable cause to believe the information discloses a violation of federal or state law.
39-2-904. Elements of wrongful discharge -- presumptive probationary period. (1) A discharge is wrongful only if:
(a) it was in retaliation for the employee's refusal to violate public policy or for reporting a violation of public policy;
Collier v. Superior Court (MCA, Inc.) (1991)
In this case we conclude that an employee who is terminated in retaliation for reporting to his or her employer reasonably suspected [228 Cal. App. 3d 1120] illegal conduct by other employees that harms the public as well as the employer, has a cause of action for wrongful discharge
Palmateer v.International Harvester Company
In this case, Ray Palmateer alleged that he was fired from his job with International Harvester after he provided information to local law enforcement authorities about potential criminal acts by a coworker and indicated that he would assist in any criminal investigation and subsequent trial. In this case, nothing in the Illinois Constitution or statutes required or permitted an employee to report potential criminal activity by a coworker. However, the court found that public policy favored citizen crime fighters and the exposure of criminal activity. Thus, Palmateer brought an actionable claim for retaliatory discharge