How do i complain to mastercard about merchant not abiding by the rules.?

my friend who lives abroad,has bought a HiFi system,but does not work from day one,they gave her another remote control ,but was not the problem,few months later the trader vanished.But the card was charged to another company.She signed an agreement for a 12 months installment.She never got the contract and the full amount was drawn from her account,and card issuer are not helpful to say the least.Can you please advise what to do next.i have written to the companies and the bank.The customers services at the bank was appalling.I had to read up on the comsumer rights and regulation,but i am not getting anyway.