Sadly based on what I am reading here, your dad has been ****** and he is now ******. The world is full of people who will happily rip people off without a second thought and it seems that this may have happened here. The reason people get away with it is they are dealing with people who don't know better or who simply forget all logic and 'do the deal' because "he seemed like a nice bloke". The added issue is older people still believe they know best when in reality they often don't and it becomes a real embarrassment.
I had a similar issue some years back with my mum who took a TV to be repaired by a local TV repair shop and was ripped off for £120. The bloke did nothing, he was bent, which I established after I paid him a visit. His shop alone smelt bent and I would not have touched it with a bargepole but my mum thought he was a nice man and he had done a couple of TV's for friends. Put simply he took my mum for an idiot, something she is not, and played on the fact, he thought, that there would be no come back.
I got the money back through some 'collections' associates but he was scum and for me it became a point of principal that cost more to fix than the payment retuned, but mum got satisfaction and was none the wiser.
I hate scum, but sadly the country and world is full of it.