Covering for a coworker means taking their shift, not lying about where they are. Know the difference.

One of these costs you something and the other costs somebody else something. Taking a shift is a favor out of your own pocket, and it is yours to give. Lying to a manager spends your credibility on a problem that is not yours, and when it comes apart it is your name on the lie as well as theirs.

Be generous with the first kind and clear about the second. You can say you will swap the shift but not account for where they are. Anybody who pushes past that is asking you to take the risk for their decision.