So, I believe the OPs friend must have got several awful experiences in his gambling life and has determined to do away with it which was why he got a fixed comfortable casino sites he only decides to bet on.
It is either he must had once been an irresponsible gambler who doesn't have a Gambling budget. He may had probably be gambling at any casino gambling sites he comes across and with the attentions to double his stakes after winning and by so doing, he must had been thought unforgettable lessions on the process. So he decides to manage himself by disciplines of Gambling in just few chosen sites which may give him the chances to set up a reputable gambling budget.
Other hand, it may be that the said casino sites he is comfortable with must has also contributed in helping him out from the pool of his irresponsible gambling. So he got adapted to those casino sites in appreciation to had helped him fixed his gambling messes up. There then he accepts that those casino sites are reputable, caring with integrity.
Just these points could be his reasons. Just a thought though.