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.
I would always say that gambling can be enjoyed as a fun activity but if someone basically gambles to earn money then that person really needs some good advice otherwise he can get addicted to it.