This is disgusting. Sorry to hear what happened but I am glad your dad had some cash on him and handed it over with no problems. If he had none or resisted, I would hate to think what would happen.
Now that said, if I saw someone do this to my nan I swear I would gladly go to prison for the pleasure of what I would do to him, and let's be honest, if you caught someone like this, what you do to them would be worthy of a long stay. Yeh it is only money, but scum that do abuse the old or young deserve special treatment.
I hope you find the [!Expletive Deleted!] and if you do, may I suggest an old medievil approach. Try tying his hands to one landy and feet to the other and give it some throttle.