Well, I definitely wouldn’t leave the unit. I’d wait for a decent amount of time and then talk to my supervisor and offer to stay until they turned up or someone else was found to fill in for her.

This has happened a few times and I usually try calling to see if they are on their way or what’s holding them up. It’s just a matter of using common sense and not rushing off because the person hasn’t arrived on time. But they had better have a good reason why they’re late.