I've checked marinetraffic.com, looking at both Ishigai and Okinawa islands, especially looking at Naha City and Ginowan City. No sign of an AIS signal from s/v Voyager.
Did you friend see the boat at Ginowan Marina, at Ginowan City which is north of Naha City? Ginowan Marina is popular with foreign cruisers. If it is Ginowan Marina, you can contact the office at: firstname.lastname@example.org
Does your friend's photo show what national flag the boat is flying? Was there a calling port named on the transom?
I cannot find a BCC named Voyager in the Forum database. And so far my searches on the Internet have found one US-flagged s/v Voyager (likely with calling port in California) but it looks about 50 foot and could not be confused with a BCC.