If you were in the line, whether he is due for break or not a regular cashier, they are supposed to clear the line before they turn anyone away.

If it had been me, I'd have said something right then and there to the GSTL on duty. You'll know the person you need because they are typically standing behind the lanes with a walkie talkie, up at Guest Service, or tending to cashier tills and such.

The best way they can fix a problem like this is if they know about it right when it happens. That's what the GSTL's are there for.