What should I do if I learn that an employee has leaked confidential company information, and would informing his manager be considered backbiting, or would silence make me complicit in covering for him?

If the information is a company secret, it is not permissible to disclose it, because disclosing a secret is a betrayal. What this employee is doing is wrong (munkar), so he must be advised and reminded of Allah. If he does not stop after being advised, he should be threatened with reporting him to the authorities. If he still does not stop, then there is no harm in reporting him to them. This is out of sincere advice (nush) and not out of instigation (fitna). Remaining silent about this is the true instigation (fitna).