Jay pretends to not know who Justin is, in front of Susan, despite actually knowing who he is. This is mostly pertinent to their discussion of "masks", or false personas, as well as Jay not being able to talk about being a wizard. (Grace would sympathize.) Jay remembers Justin as the fire-monster cheerleadra guy, and even though she hopes Susan knows about magic, she can't be sure.
But there's another reason, even if Jay doesn't remember it! Jay goes to the same school as Justin, where she [acts like] a huge gossip. The big turning point of Justin's life (in the comic so far) is being outed as gay to the entire town via the rumor mill during his freshman year. (Reportedly even his parents heard about it at the grocery store.) This led to extensive bullying and isolation, which in turn eventually led to him working at his uncle's comic shop and learning Anime-Style Martial Arts with Elliot and Nanase, and eventually making more friends. But he still resents the rumors that outed him initially. So Jay, by pretending to not know Justin for "keeping magic secret" reasons, is incidentally obscuring that she almost certainly had a hand in spreading those rumors about him 3 years ago, even if she doesn't remember the rumors or realize that it's the same Justin.