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.
A statement can only be considered defamatory if it is made to a third party and it is not 'fair comment'
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.
Sorry but i strongy disagree on this one I have a training captain that flys with us and has around 15,000 hours but has never held an instructors rating, I have an instructor with less than a 1000 hours, who is better placed to check out a PPL? Its not the rating or licence you hold its your competence and experince which are important--remember the Invicta Vanguard accident at Basle--The captain had failed his intial IR 9 times and the FR shewed he banked the a/c in cloud at angles of upto 60 degrees and thats even before you consider he did a NDB let down at the wrong beacon and end of the airfield and flew into the mountains.