Whilst I still think there is far more to this story than your mate or perhaps you are saying ($5 stakes and running up $12k, several trips and small transactions a time, plenty of time to get out of the game – even that blog makes it clear they knew it was a scam very early on, they thought they were going to make money etc) if your friend thinks the bank (and it is the bank not Visa etc) has acted unreasonably he needs to follow the formal complaints process.
If the bank still reject his complaint he will need to complain to the Financial Ombudsman Service and get them to adjudicate. Basically he would have to explain that whilst he authorised the transactions he felt coerced in to it and that the bank had a duty of care to prevent such a large sum going out in a large number of small transactions over a relatively short period of time.
Its a long shot but his best hope.