abandonment laws

My wife is currently employed in a nursing home, as a cna. She is scheduled for 12 hour shifts. The don of this facility has on several occassions told her she has to work over,sometimes its 15 minutes and at times it is 16 hours total. Then to return the next day for a 12 hour shift. This is due to them not having proper staffing and scheduling, if she tells them she cant, which is not verry often,they tell her if she leaves after her scheduled 12 hour shift she will be terminated and her liscense pulled by the BON for abandonment of residense, due tio them not having night time staff.They force her to stay over and let their other employees go home to their family when they refuse to stay over. My worry is that she will become unemployed and not have a liscense if she sais no, is this legal for a employer to do as a nursing facility in oklahoma?