Usually, when you interview for the job, they tell you if you are expected to work night or weekends.
She should not have taken that job.
Maybe she actually had something in writing indicating that she would never have to work Sundays.
If you tell them that you won’t and they agree to that as a condition, then it is their bad. I worked at the Grand Ole Opry for a summer and made it clear when being hired I wasn’t working Sundays. They agreed, and I did have to remind the person who made the schedule once, but that was it.
If they agreed and accommodated initially, then it was fine for her to take the job and their problem when they changed.