Any information linking what HS Anon had said about Greg's high school years and what we know of Greg's time in high school (from what he had said in archived websites) could possibly be considered evidence for or against Anon. It could be considered evidence proving HS anon was being truthful in his account. It could also be possible HS Anon was a troll and got some of his information from these websites to seem more truthful.
Suspiciously, all of the images HS Anon provided on the image boards as proof of knowing Greg are all archived on a 2006 version of the Onision site. HS Anon mistook Alicia, Greg's ex-sister-in-law, for one of Greg's biological sisters. On the archived page, Greg referred to Alicia as his sister. (Source)
SomeGuy827 claims that before he interviewed Anon, he showed him proof that he went to the same high school as Greg.
On August 29, 2015, "HS Anon" came to an image board thread about following Onision and his drama. He posted a long, eight paragraph tell all about his supposed years of knowledge on Greg based on their time in high school together and from mutual friends. He began answering questions from other board members and began posting old photos of Greg. He claimed he had these photographs because he and his friends would save images of Greg from his MySpace as a joke. He answered question for hours until he said he would be back to answer more questions the next day. He did not return and instead, contacted Some Guy for an interview.
(Archive of image board conversation coming soon.)
On August 30, 2015, SomeGuy827 uploaded his interview with HS Anon to Sound Cloud. (Source) He distorted Anon's voice at his request to hide his identity. For over 40 minutes, HS Anon answered Some Guy's questions, told stories he supposedly witnessed or heard of through friends, and gave his opinions about Greg.