He certainly had some kind of trigger for this. For someone to go more than a year without playing and not resist the addiction, something very serious must have happened... it could have been anything from an emotionals problems like stress, loneliness, or even boredom to something more serious, like a financial problem (as silly as that may seem).
Sometimes, even being to close friends who plays games could have influenced him again... it's hard to live with someone who plays, hear them talk a lot, and resist the temptation.

And how avoid it? Well... by recognizing and avoiding these triggers, right? Keeping your mind and body busy to avoid depression and idleness, staying away from people who have a gambling habit if you don't want to play anymore, seeking psychological support to stay on track and maintain a positive mindset... the same older story... I don't think there's a secret, only the hard part is doing it.

The recovery process from any addiction is long, unfortunately our mind does not help us and the main problem that every addict faces is exactly these relapses... I think that support from friends, family and a professional is essential to stay at right path, even after years.