|The whole situation screams STOLEN loud & clear and the onus is on the pawn shop.|
Any pawn shop operator with any experience would have seen the machine boot up and asked for passwords from the seller before parting with their money and so should your client.
Take it back and demand the money back for it. At least in this country, pawn shops are required to obtain ID from any seller they buy from.
Goin' Fishin' (Some day)