I'm reading card as a bank card. Any opinions?
It's the only meaning that makes sense to me. I think CTH has some speculation the remark revolves a SIM card for a phone, but I don't see that being a "hustle" or any particularly damaging thing to have around. Not that a stolen SIM couldn't be trouble, they are, and pretty easy to trace what with cell tower records and the like. But Martin had a phone that worked.
Oh, maybe it is a fake or stolen ID card.
Whatever it is, it's shady. Shady enough that his shady friend urges Martin to ditch it. I don't think being caught with fake ID is nearly as much risk with the law, as being caught with a stolen ID or stolen bank card.