The BP machine is not measuring electrical activity. It's measuring the time between the pulses (systoles). If the timing is irregular it will show that symbol for an irregular beat. There's no way to tell based on that what actually is going on. There are dozens of electrical issues that can create an arrhythmia or irregular beats (pulses/systoles). systole is just the a term meaning the heart's ventricles squeezed blood creating an outflow that you feel as a pulse.
If you feel her pulse on your own, is it irregular? a regular rhythm to it, then a quick two beats followed by a pause? Those would be ectopic beats either premature atrial contractions or premature ventricular contractions, or some other stuff that could be coming from the nodes or junctions that would have to be caught on an EKG to figure out. Most usually harmless.
The other stuff you describe (dizziness, headache, tiredness, breaking out in sweats with heart pounding, hands shaking) possibly fits under anxiety disorder and would also create the heart palpitations.
personally I wouldn't read too much into a borderline EKG. EKGs are often false positive or even worse, false negative meaning they don't detect an issue. EKGs are a quick and cheap test but take follow-up testing of more value if it points in a direction to figure out what's up.
If her heart, physical, and blood work all check out you will be left with endocrine issues (lots of blood work)
p.s., and if endocrine checks out, and everything else is good, diagnosis might be anxiety disorder / panic disorder. Work it out with her GP family doc. They see this all the time. Getting treatment for it whether medication and/or coping skills will be the way to go,