It's not the not telling anyone or the freaking out. It's the whole fanon thing that after they get back together, Justin feels guilty and Brian feels mistrust of Justin about the Ethan thing. There is NO canonical basis for that, none at all. I can understand it in fic written in the S2/S3 gap, but for a post-S4, no.
I tend to think Justin would actually have told Brian the minute it happened... they were pretty tight in canon at this point. But you're right, it's arguable. What I love is how she brought it home by having Brian echo the cancer dialogue to make his point that it's the same. Genius.
Bunny told me to get the fuck out of here before I ruin one of my absolute favorite stories for myself and you, LOL, so I think I will take her advice.