The symptoms she is having sounds like she is having some kind of arrhythmia
on exertion she needs to speak to her GP ASAP who could ask her cardiologist to do a 24hr tape (is worn home monitors the heart for 24hours) or if she is very breathless and dizzy when it happens or it takes very little exertion to bring on the symptoms take her to your local accident and emergency dept, if she is having some kind of arrhythmia taking her in while she is actually experiencing symptoms means she can get an ECG at the time so it will actually show whats going on. If an ECG is done once the symptoms have settled it may not show the problem.
Some arrhythmias, espec if the rate is very fast can be harmful if they keep happening for prolonged periods
and in the worst case scenario could lead to a cardiac arrest
, I'm not saying this will happen but you wouldn't want to risk it.