I would say that expensive and long-term rehabilitation is not necessary for recovery. In this case, it is more important that the patient himself wants to recover and leave his harmful addictions. However, if money and time allows you to go through rehabilitation, it is unlikely to hurt.
It will be useless for a gambler to spend many of his time and money into a rehabilitation center if he himself doesn't want to recover.
For me, it would be better if you just guide the gambler and let him recover if he really wants too. Consulting to an expert will be helpful too but don't spend too much in rehab. There are some who can just recover from it and there are some who can't do it himself unfortunately. They will just stop if either they don't have money to spend on gambling or something terrible that is related to gambling happened to them.
The problem is that because of people's addiction (both gambling, alcohol, and drugs), their families usually suffer, and therefore the isolation of such people is at least a temporary measure so that families can at least have a little rest and calm down. But the path to full recovery is very long and there is no single correct recipe for how to beat addiction.