Any idea what type of arrhythmia? I travel with my arrhythmias, no problem. If he's having v-tach, he may have to postpone the trip until it's stabilized with meds or worst case scenario, an ICD. And good advice for anyone, if you're going to be on a long flight, it's always good to get up and move around every hour just to keep the leg circulation going.
No comment on the travel insurance company subject, but I can say I have flown a lot and suffer from AFib. Here I include flying for upwards of 12 hours on a single flight and there was no specific effect on my heart that I am aware of.