I agree with the others.....wait until he goes out,change the locks and leave his belongings in a secure location for him to pick up at some time in the future.
If your girlfriend goes to her local police station and explains the situation before hand they may go to the house at the time the ex is expected to return to prevent any violence.
Unless he stole the cards from her and somehow conned the pin number out of her or the CC company then she is just going to have to pay the bill/debt herself.
As ummmm, kunni and others have told you......either visit him with some family/ mates and persuade him to leave the house and your g/f alone, without resorting to violence, as the sheer presence of alot of you should be enough, OR.... call the police and let them look into it, and deal with it.......that way, at least, its on record.
daffy and others suggestion of changing the locks and getting his stuff out in a secure location, is also very good, but if you dont get the police involved to be there on his return, he is more likely to make a big scene and who knows what?.
im not the ex, im the one she wants to be with but she is too stubborn to do anything and doesn't want me involved but im not letting it rest cus i can see and hear what its doing to her.