There are a variety of heart rythm irregularities which range in severity from harmless to annoying to potentially fatal.

If it is of the harmless sort, it will not be a problem. But if he's not sure, he needs to see a doctor asap to find out.

If it is not-so-harmless, beta-blockers are a common treatment for certain types and plenty of pilots are certified with the use of those meds.

Some things aggravate the harmless arrhythmia...caffeine and fish oil diet supplements come to mind.