Her disrespect won’t make him want to cover any last-minute shifts in the future. 

Are you obliged to cover your coworker’s shift if they call out at the last minute? Many bosses believe that being unable to come into work at any hour of the day is a personal slight and shows a lack of work ethic. I don’t really think companies that pay pennies should be so offended that their employees aren’t willing to cancel their personal plans to make an extra $40. If you’re making six figures, you’re probably more likely to do whatever your boss tells you to do, because you don’t want to make them angry or put yourself at risk of losing your job. You get the level of dedication you pay for, and you can’t expect your employees to respect your time when you don’t respect theirs. If it’s really such an emergency, the manager should cover their employee’s shift.