I agree with everyone that if a charrie doesn't hear directly, then they don't know. But sometimes I've made exceptions. For example, at the beginning of this term, I played with Felix at the opening feast. Even though Marigold and Toby hadn't met directly, we both assumed that they would know each other after being in the same house and year for two years previously. So basically, if it really really really makes sense for your charrie to know something, it's okay.
Also, I'm really out of the loop
anyway, so my charries and I are pretty much on the same foot here.