Quote Originally Posted by thalassa View Post
Something else to consider is that she might have a known issue that the supervisor is already aware of--inconsistent transportation, having to wait on child care or a spouse coming home from offshift to watch kids or bring the car for her to use, or that she has a health issue that you are unaware of, etc. It might be that they've already had this conversation and are willing to work with her (I'm not meaning to sound harsh here), its none of your business why she's late. In which case, the problem is how she's being scheduled, and there isn't anything you can do about that.

The thing to remember is that you are not obligated to stay late to cover for her. If you don't want to stay late, there's nothing wrong with saying you have plans and can't stay (unless you want the money).
I second this. I'd talk to her about it and let her know that it's disrupting your own schedule. Maybe there's something you can work out together, or maybe there's something you both can work out with your supervisor (like, your shift starting half an hour later or something).