If the pilot to be 'checked out' is already licensed to fly the type in command then 'instruction' should not be necessary therefore there is no regulatory requirement for an instructor.
Ask the the aircraft owner/operator to verify his, and any insurance company, requirements. It might be that the 'checking pilot' does not even need to be a current licensed pilot at all. It might be a condition that he or she should preferably be an instructor. It might be that a so-called 'check-out' is not required at all.
Whatever such non-regulatory requirement, when a pilot who is qualified to fly the type in question in command is being 'checked' by another pilot it is very important to agree beforehand who is pilot in command and thus prepared to take responsibility for the flight. Only one pilot can be in command (and log PIC) at any one time.