The only "authority" a check pilot needs (even if he just holds a PPL) is a notice from the owner saying that he is authorized to conduct check flights, on behalf of the owner, for the purpose of determining whether a prospective or recurrent hirer meets the qualifications and flying ability/technique that the owner demands.
The problem with such simple thinking is that the checker lacks credibility and if the person being checked disagrees then it is a 50/50 argument.
You could never let anyone else know that you did not think the pilot was up to scratch as such a statement could be defamatory and as a private pilot with no credibility in checking or training you would not be off to a good start.
Then of course you are in a catch 22 situation - if that person goes and hires your neighbour's aircraft and wrecks it, your neighbour may reasonably ask you why you failed to let them know or why you failed to record the poor performance in the pilot's logbook.
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The simple situation is that unless a proper system of credible checking is in place then there is no point in having a 28 day checkout requirement as it is meaningless.
Getting one PPL to check another lacks credibility and performs no function. Having an instructor check a pilot against the required standards has the advantage of credibility.
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Only if the designated check pilot is listed as PIC. But we're trying to convince you that it's perfectly legal, and practical, for the prospective hirer to be PIC and as such pay for all the costs of the flight.
If you say that the prospective hirer can be pilot in command then you can not say who the passenger that they carried in the front seat must be. The loading and everything else with regard to the flight is the responsibility of the pilot in command and they can quite rightly decide not to carry anyone............after all you have permitted them to be pilot in command and the decision after that is their's and their's alone.
Of course it is silly at the least to say that you can not be pilot in command of the aircraft you have not flown for the past 28 days and then say that on the 29th day not only will you be pilot in command but you must also take this other person with you without the option of cost sharing..........One might ask if the 28 day rule in that case was simply a free flying excuse for the other member of the group.
Regards,
DFC