It's rather common when a group of friends are introduced to gambling gatherings that at least one of them will end up addicted. I've seen it on a few occasions. However, I don't find it necessary for the rest of the group to quit gambling, especially if they have full control over it and it isn't causing them any issues. Certainly, they shouldn't encourage it when he's in front, but eventually his compulsive behavior will lead him to start borrowing money and going into debt in a mere attempt to recoup his losses, a common mistake or misconception compulsive gamblers have.
The whole group quitting, although not necessary, is unlikely to have an effect; he has a mind of his own, and he's free to gamble with or without his friends. They need to assist him by seeking out a professional; otherwise, he'll end up screwed before he even realizes it.
I understand - its hard. Seeing a friend spiral is hard, but I believe in balance. When done safely, gambling may be fun. Rather than cutting off something people appreciate, establish lines when necessary. If I can restrict my gambling, excellent, but I must remain watchful. Setting boundaries, especially for vulnerable people, is whats needed, not quitting. Instead of stopping the activity, we should change how we use it in delicate situations.
Helping our friend is vital. Professional help and a supportive environment are essential. Gamble properly without triggering or enabling his compulsive behavior. A delicate balance between personal freedom and collective duty is needed.