If my friend is also a gambler, then I probably would suggest online gambling.
More chances that your friend would already know about some well established fiat casino and would not be willing to budge from it and go for bitcoin casinos. So it will end up in that bitcoiner vs nocoiner debate once again and invariably you will end up spoiling your friendship.
But if my friend is looking for something just to kill time, then I wouldn't suggest this, unless he/she personally approached me about it.
I dont think you should suggest gambling to a friend to kill time. People have the habit of judging everyone and so your friend might take you as a lower standard than you because you gamble - this might seem cruel but it is the truth with many people. You dont want to put yourself in a bad name just because of your gambling so it is better to keep it to yourself.