I've read 91.715 and stand by my original statement.
Well, you are misinterpreting what you're reading.
91.203:
§ 91.203 Civil aircraft: Certifications required.
(a) Except as provided in § 91.715,
no person may operate a civil aircraft unless it has within it the following:
(1) An appropriate and current airworthiness certificate.
Each U.S. airworthiness certificate used to comply with this subparagraph (except a special flight permit, a copy of the applicable operations specifications issued under § 21.197(c) of this chapter, appropriate sections of the air carrier manual required by parts 121 and 135 of this chapter containing that portion of the operations specifications issued under § 21.197(c), or an authorization under § 91.611)
must have on it the registration number assigned to the aircraft under part 47 of this chapter. However, the airworthiness certificate need not have on it an assigned special identification number before 10 days after that number is first affixed to the aircraft. A revised airworthiness certificate having on it an assigned special identification number, that has been affixed to an aircraft, may only be obtained upon application to an FAA Flight Standards district office.
He doesn't have that, the aircraft is unregistered. So it kicks over to 91.715:
§91.715 Special flight authorizations for foreign civil aircraft.
(a)
Foreign civil aircraft may be operated without airworthiness certificates required under §91.203
if a special flight authorization for that operation is issued under this section...
I doubt he has that special authorization as he is not owner and isn't even sure if it's foreign registered. Hence it's not a legal operation. What's an operation?
§1.1 General definitions.
Operate, with respect to aircraft, means
use, cause to use or authorize to use aircraft, for the purpose (except as provided in §91.13 of this chapter) of air navigation including the piloting of aircraft, with or without the right of legal control (as owner, lessee, or otherwise).
Nothing in there about flight or intent to fly. I can't make it any clearer than that.