Most fetal arrhythmias are benign and go away after birth. Usually, because there are a few potentially serious ones, we have an ultrasound (fetal echo) of the heart done to rule out more serious problems. The most common cause are premature atrial contractions. I've taken care of a number of mamas with fetal arrhythmias and they have all turned out to be normal babies after birth. Often, the arrhythmia has been more pronounced, and then less noticeable later on, and sometimes it persists until delivery or even after.
As an aside, one problem I've had with fetal arrhythmias is that an electronic fetal monitor often cannot trace the heart rate accurately due to the arrhythmia. With fetal arrhythmias during labor, we've had to use doppler and charting what myself or the nurse can HEAR because often what the monitor reads out is terribly innacurate. Make sure her care providers are aware of that, because you'd hate to see a monitor read out relied on if the arrhythmia was making it difficult to trace.