I suppose the balance is to be found in the way that one handles/expresses this.
The best way to handle this is to explain in plain English to your potential customer that it is somewhat difficult to ascertain where they are at from their logbook, and that you would appreciate it if they would be able to ask for the training records from the previous school. Also get the former school to stamp the logbook, so as to certify that all entries were correct.
There is no need for any sarky comment, nor is there the need to brag about how much better your school is! Especially not in front of the client. You act professionally, and SHOW the client how much better your school is, don't tell them - words mean little to people who know nothing better, and will never know until they have seen it for themselves!
Then go for a flight of an hour and a half or so, and find out how much or how little the customer actually knows. Look at what appears to have been covered well, as well as where there is room for improvements. Make a decent entry into the new training records stating all aspects of the handling skills (or lack thereof), similar to an overall evaluation of skills similar to one you would do before sending a student for test and put a plan of action to get them up to speed. Have a look at where they are with regards to groundschool, and again evaluate for yourself if some of the key concepts have been understood, make another plan of action for the groundschool.
It really is that simple, and you'd have a very happy customer at the end of the day, who may well keep renting from you for years to come!
Not going to defend my flying school, we have been around 10 years and still have cash in the bank!
Are you saying that this incident was at your school? Or that you have similar take on customer services?