What could be the reason for his return and what can be done to manage such situation?
Generally, everyone has their own reasons for returning to gambling after a break. If they previously stopped because they were busy with work and didn't have time to enjoy gambling, then when they have free time, they can return to gambling without any problems. I see people like this as responsible gamblers, of course only if they gamble just to enjoy free time and have fun. Moreover, situations like the one above are not always a problem, especially for those who are responsible gamblers.
You are right at this point because if actually any one can boost if being a responsible gambler that gambles for the fun then there is a possibility that at some point gambling maybe not really be what is needed to be included in his fun life, he can only come back when he or she deem it necessary to, this is a personal experience for me and of course if priority is given to gambling for fun and gambling responsibly any one can quite and not quitting completely and come back for one for one reason or the other, it can hard to leave anything that is already part of you provided that in your overall judgement it is harmful to you.