He can't take it back to the shop once he has gone direct to the manufacturer, it's either one or the other, you always try the shop first and if they try to get clever then you go direct to the manufacturer.
Which company is it mate?
Explain them your situation and it is impossible to be a fault due to misuse,have you already told them you are not accepting their explanation? If you get no joy ask to speak to someone with more authority, also it is best to get hold of the company directors details (this can sometimes be found online), you can use this to your advantage when dealing on the phone, tell them you will be writing to Mr or Mrs such and such regarding their after sales care, but first of all always get the persons name you are dealing with on the phone, this must be done nice and early, just bring it up in converstation, as in "And I am speaking to?" This way you have a person to point to when dealing with any complaints.
If they still try to get clever tell them you will seek legal advice if they do not cough up, also explain to them that you are avid writer to quite a few PC magazines and I am sure some of them would like to hear about the situation.