Invited to Query, but Where Do I Stop?

So to me, this isn't exactly a solicited query, but it's not unsolicited either. You specifically were given her name and email address, so obviously you should be sending it to the email, not through postal.

I would send the email, title it Query: Title/Genre - Referred by (original editor). And then include what she typically asks for from unagented authors. Mention the situation in your opening, how (editor) gave you her email address because she thought you would be interested in the novel. Then mention that as per her guidelines, you are attaching your query, synopsis and first three chapters. Typically, "querying" an agent still means following their guidelines. I think it would be better to send more than less in this case. It shows that you actually looked her up and didn't just say "oh, here's an email address! Here's my query letter!" and that you looked into her background/wishlist/etc.

Obviously not an agent, but in any circumstance I would imagine sending more than needed would be 100x better than not sending what was required. I've seen may agents on their pages say that if not all materials are included they won't even consider the query.

Anyway, that's definitely what I would do. Good luck! Pretty cool you were referred to another editor.

Title the email: Referral by (friend's name). If you put Query in the subject line of a person who doesn't normally accept them, you're going to get trapped in a spam filter. Then you put the query in the body of the email, and attach the synopsis / first three chapters.

HOWEVER, keep in mind that if you're also seeking representation, you could be burning a bridge if this is large publisher. If you query someone, any agent you sign with can't query they again on the same project.