An irregular heartbeat is very dangerous and she shouldn't be doing anything to make it go up because an irregular heartbeat makes blood clots and causes heart attacks and strokes; I know, i've had both due to an irregular heart beat.
The previous poster is right that other things can be wrong with her heart, such as valve problems. Don't believe them if they say her heart is "fine"; this is not normal and is indicative of further problems. Even cardiologists give a minumum of tests; what did they test her for?
Hawthorne berries are helpful for rapid heartbeat, but it's just a bandaid...don't think it will solve her issue; she needs to find out exactly what is going on.