Let's assume that GIJoe's dad actually did monitor his emails/PMs (instead of just saying he did to get other parents off his back), and let's assume that "monitor" means reading through every single one of them.
If this is true, from strictly an email/PM perspective, he would see his son buying comics, then later coming back and saying they are not the grade stated. Not sure if one can make the jump (without hearing conversations b/n the two) from that to knowing his son was not telling the truth about stated grades.
*** Note: I did not read through every post here, nor track through every FB message, so if there is definitive evidence about the grade negging, I will eat crow.