The information you provide seems to my reading to conflicted. That is Atrial Fibrillation normally causes a high (too high sometimes) heart rate that is also irregular. I have never heard of a HR being too low due to AFib.
That said, I assume your father is on a heart monitor and, perhaps, someone said he may have periods of AFib, which a monitor can detect. The monitor also detects a HR that is low, and one that is normal. It is a monitor.
While some doctors may be able to identify AFib by listening to the heart, I believe it is necessary to check the electrical signals, such as with an electrocardiogram to diagnose AFib. There is a unique and specific waveform for AFib. In most cases, I believe, AFib is not in itself dangerous but it can elevate the chances of a stroke. Perhaps that is the connection to the diagnosis of AFib.
Tell the nurse you want to talk to the attending physician. Find out what time they come by in the morning and be there before just in case. You have a right to an explanation of his condition, possible treatments and probable outcomes. He should be seen by a cardiologist, pulmonologist and maybe a vascular doctor for the stroke.
The way things are today, you have to keep a close watch and make sure they are carrying out doctor's orders. I had to question my Mom's medication more than once. Is it a small hospital? Maybe he needs to be moved someplace where they have more resources.
Stay strong and demand answers! Good luck.