So far I have never heard or known of doctor who advised patient to engage in gambling activities and of course doctor would only suggest more beneficial activity such as exercising, this would be very useful alternative activity.
That doctor might be a gambler himself and finds gambling activity as stress relieving activity for himself that's why he might be suggesting the patients to gamble in order to reduce stress. 
In real world prolong gambling can in fact increase someone's stress levels and that's why most people recommend to gamble within limits.
Some people who get addicted to gambling often spend countless hours to place bets and if the bets turn out to be losing ones then such people get addicted and even face more stress from the activity.
Gambling will be beneficial for those who can control themselves and are able to take responsibility for their gambling, if someone is easily stressed or has such a disease, it can be said that he cannot control himself, how can he gamble and relieve stress if he cannot control himself even if he is forced to gamble, I think it will have more adverse effects on the patient such as impulsive and compulsive addicts.