If you or your friend is poking around a "found" usb with btc data...on a computer that you use regularly and/or with your personal wallet on it,  there's a really good chance that you'll soon be back here posting about how your wallet was hacked.
If I was you and had a wallet on a computer that you plugged that usb into, I'd move the funds to a different wallet on a different computer of phone, STAT.

You dont mention it, so I assume you didnt do this: anytime you find a USB and want to see whats on it, you should use a "live CD" boot on your computer.