Great that they found it in time.
An arrhythmia can be life-threatening if NOT found, but it's often correctable these days with some variant on a pacemaker. Keep in mind that the heart runs on electrical impulses; this is a bit like a short-circuit. It also looks as if this was one of the kinds of arrhythmia that can be improved by cauterizing an area in the heart.
I'm not a doctor, but I had a cardiologist in the family, so I have some small knowledge of these things, and I hope this helps a bit. Just to reassure everyone, this is very different from what Sergei Grinkov had, which seems to have been a genuine, early-onset atherosclerosis, or closure of an artery by cholesterol. It's also very different from a heart valve malfunction, which usually needs significant surgery (a valve replacement or repair), but which is also more curable than the closed artery. The arrhythmia is not related to any bacterium or inflammation. Some people just have it or develop it.
Depending on the situation, this could just be a bump in the road for Nick. It's actually not uncommon in young people, and it tends to show up in the very athletic because they exert themselves so much more than average. Some kids collapse and do not end well, but many these days are found in time, and they just proceed with their lives after treatment. The sudden appearance of an arrhythmia during a sport or game is why many sites in the U.S. now have defibrillators so they can immediately jolt the heart's electrical system back into smooth running.