Somebody feel free to correct me if I'm wrong, but I thought that Aviva said that Cody asked if Nana could spend the night, and then Aviva told Cody that it was fine and to put her in Harrison's room. In other words, Aviva was going off of the last communication that she had with Cody, not on any firsthand interactions with Nana.
Assuming that Cody simply didn't bother to update Aviva on the fact that Nana left directly from the party, and given what George said in response to Aviva's questions about where she was, Aviva might not actually be intentionally lying.
Also, call me a pervert, but am the only one slightly suspicious about Nana's responses? Girl had some answers at the ready, and never even asked Ramona why she was asking for all of those details. I'm not saying that she hooked up with George, but maybe she hooked up with Cody and George watched or something? I'll go bleach my brain now.