A couple of things:
1. I hope you are charging your client at least $100 to work on this problem. I get these cheapos with their odball problems all the time. If they had paid a fair price for stuff in the fist place, they would not have these oddball problems that they now want someone to fix fo free. Bottom line is that your professional time is worth money, and I hope you are not just giving it away (aka slavery and/or the poor house). If the client is not willing to pay a fair price then you should "ring it up" as no sale.
2. If this guy wasn't a cheapster, he would have his POS terminals furnished and installed by a local dealer and would not have this problem.
3. If he was buying from an authorized dealer, he would be able to get replacement parts. The guy who sold him this POS terminal probably won't furnish a replacement cable because he doesn't have any because he is not a regular dealer of this equipment.
4. Contact the manufacturer, who may or may not help you either with a pinout or a replacement cable.
5. I would follow up the suggestion that you test each conductor for diodes. Most inexpensive VOM meters contain a diode tester. If yours does not, then beg, borrow or steal a diode from Radio Shack or somewhere and put it in series with your continuity tester. I would also check for resistance. I would check for capacitance if I knew how. Anyone?
6. Your best advice is to just tell the guy you can't help him. There is no money to be made with this type. He is probably lying that the cable was left out when he bought the thing. Otherwise he would have returned it faster than a hot potatoe.
7. What is this cable supposed to be connecting? Standard Ethernet uses pins 1,2,3,6.
