What could be the reason for his return and what can be done to manage such situation?
You can't really be sure why he return to gambling but I think financial problems can be a good reason to revert back to gambling after leaving for while. Difficult times is reason enough and sometimes it could be peer influence but doesn't happen to a lot of people after leaving they will be able to bear the pressure.
Returning to gambling isn't the main problem, but returning with the same mentality as before.
Most of us quit gambling when we got addicted, but the abstinence moments allowed us to heal, have a new orientation and replaced the addiction with a new activity and finally returned a more disciplined gambler.
If the gambler has solved the reason he left and is prepared to gamble responsibly, then he's fine.
Often times we return to gambling when we get bored of refraining from it or becoming interested again from association with gambler peers
Exactly, retuning to gambling is not really a problem but having the same mentality as before should be the major concern. I think it will be difficult to have the same mentality as before after quitting for a while say about a year or more, even an addicted gambler will not be as obsessed with the game as before even if he return to gambling again.
If the gambler fails to replace his mentality with a positive one then he definitely is safe but in the case where he still returns with the same mindset I don't think there will be much difference, he is still likely to end up getting addicted again.