The question read:
I am thinking it doesn't matter if it's a lie or not and you're allowed to voice your opinion, ...
Under US (and I think also Canadian) law, it very much does matter if such a statement is a lie, the truth, or an opinion (which is neither).
If a statement is a statement of fact, and is false, it might be grounds for a defamation suit (in this case probably slander, rather than libel, in those states that have preserved the distinction).
A statement of opinion is not a statement of fact, and is not taken as either true or false. However, sometimes a statement framed as a statement of opinion is treated as a statement of fact. "I think it is clear that Joe murdered Fred" would be treated as a statement of (purported) fact in a defamation case.
To prevail in a defamation case, the plaintiff must show harm to his or her reputation, unless the defamatory statement falls into one of the categories considered defamation per se Traditionally these are:
- statements or suggestions that a person was involved in criminal activity;
- statements or suggestions that a person had a "loathsome" contagious or infectious disease (normally a sexually transmitted disease);
- statements or suggestions that a person was unchaste or engaged in sexual misconduct (in some jurisdictions this applied only if the person was a woman);
- statements or suggestions that a person was involved in behavior incompatible with the proper conduct of his business, trade or profession;
Specific legislation has modified these categories in some jurisdictions.
A statement asserting that a co-worker was incompetent might fall under the fourth of these categories. A statement that a co-worker cheated his employer almost surely would fall under the fourth category.
In the absence of defamation per se a successful defamation suit would need to prove specific damage to reputation, which would include proof that the person defamed had a reputation above rock-bottom before the defamation occurred, and it was harmed by the statement.
Negative statements of opinion about a co-worker would not be grounds for a defamation lawsuit, but might violate the employer's policy and be grounds for discipline or even dismissal.
In the US, the First Amendment's protection of expression now makes defamation harder to prove than in many other countries.