Unless you are able to prove that your cardholder was complicit somehow in these transactions, or benefited from them (both unlikely, in my view), I think your belief they are unauthorized is correct.
I also think you would be very well justified in refusing to issue this guy another card. What he did was clearly negligent.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8