I don't think a specific reason is necessary for someone to start gambling again. Often, just a momentary urge or seeing someone else playing and having fun can spark the desire to try their luck again. It doesn't have to be anything grand: in some cases, simply watching someone else gamble is enough to rekindle that desire.
In OPs friend case this is not it, he was a gambler who lost and quit but couldn't move on and ended up coming back only to make things worse, I think he is clingy to the past memories, something I fear most in gambling, your past can easily come back to haunt your future.
If this person can't move on but pretend to have move on it would hunt them again, he wasn't fully prepared to make a comeback if not he wouldn't have turned into an addict and still lose everything.
It's like having a old girlfriend who turned to your ex and after several months or even years have passed you still can't move on, when else you guys see each other again the same old feelings comes right back at you. Most gamblers don't really move on, they are too scared of losing money and they can't do without gambling.