Verbal conversation and demands have no ground to stand on, in the court of law unless backed up by a physical evidence. I think what Dave is doing correct, he can then gather all of the evidence together should he need to use it. Black and white is by far more substantial v hear - say.
An email with the demand has been sent, a verbal (phone call) followup perhaps isn't a bad thing, but what are the chances, that the bloke he wans to be chatting with, will wave his colleague, that "I am not here" or "he's out the office" or "he's in the meeting"
A visit into the store, with a demand to have his money back, maybe a good thing, maybe not. I am a cabinet maker after all and most certainly not a lawyer or a legal advisor.
Perhaps should the visit into the store not go the way Dave is intending for it to go, then a verbal notification of "a video recording for the purposes of evidence will commence" and start recording using your phone, once again, the moment the video recording starts, make sure to mention that for the purposes of transparency, you are gathering video evidence... following by a request for an explanation, as to why is your money and request for a refund are being withheld and/or ignored.