If most of what they did was gamble and pressure each other to then it might not be much of a loss for op. Also I'd think trying to get past the addiction would be more important than those friends at least for the first couple of months/years. If gambling was a big part of their group/life then op or the group might feel rejected if they go back too soon too.
2. Contact the Gambling help section in their country, you can even talk online to the experts
3. Make sure you do not use the website by collectively banning them from your phone/account.
The medical help could point in this direction but I think addictions are easiest to replace with an enjoyable hobby (as long as you pick the right one - especially if there's similar aspects to the previous addiction; such as the social aspect op mentioned).