As soon as he describes the charge(s) as 'unknown' to him, his card issuer is going to cancel the card. It's compromised as far as their concerned.
I ran into what I thought was fraudulent charges a few months ago. One Monday afternoon I got gas and used my card. At the end of the transaction, the pump said "Please see cashier" so I thought there was a glitch with the payment. I got in the car and checked my bank app and it showed a pending transaction for the fuel I just purchased so I figured, it's okay after all. Then I saw a charge for $12-15 from "Drunken Monkey, Celina OH". What ?? Never been there, never heard of the place.... Googled it and it's a bar/restaurant that closed 6 months prior. I called the bank, told them I didn't recognize the charge, and they told me the policy is cancel the card and re-issue a new one. Later that evening, I Googled "drunken monkey" more and found a Facebook page, then found posts related to the former owner and his carnival ride and food business. Turns out the funnel cakes we bought at our little town's downtown festival were from this guy's business and he was using the bar's credit card processing account ! I got in touch with my bank and they said they can 'stop' the card's full cancellation if it's within 24 hours or so and I was able to. I even told them I figured out what the charge was and they un-reversed it. Oddly enough, the fuel transaction never showed up on my card. I suspect what happened was they tried to process it, it was declined because the card was cancelled at that point. I tried calling the gas station's customer service and credit card department multiple times (I kept records) to pay for the fuel but after long hold times or getting run-around, I gave up...