BE is not for beginners. I often get inquiries from enthusiastic horn players who are newly coming back to horn. In the past I've sold the book to a few horn players who had only been playing for a few weeks. It's worked well for most, but not everyone.
In my opinion BE works best if the horn player already has a stable, not necessarily fabulous, but stable embouchure. I now tell any brand new come back players to hold off on starting BE until they've been playing long enough to first get well acquainted with what their natural embouchure is.
BE has to have a starting point, something to work with, something to build upon. If a player is unduly insecure in their regular set up, BE can cause confusion.
BE works by challenging the embouchure to function in two extreme set ups -- the rolled-in and the rolled-out. Over time, the bits and pieces of technique learned in the challenging exercises eventually influence the player's regular embouchure. If the player doesn't have an established regular embouchure to rely upon, they can easily become confused in their approach to every day, regular playing.
So to be perfectly honest, I'd get a little more spending money for Christmas if I sold the book to everyone who asks, but for best results, I want brand new, fresh come back players to wait a while.
How long? Well . . . if you've been playing for a year or more, I believe you're ready. If you've been playing for less than six months, are still fooling around with different embouchure settings, horns, mouthpieces, etc., you should probably wait a few more months before starting BE. First, get yourself acquainted with how you play your instrument, then order the BE book. (But don't wait forever or you'll miss out on the opportunity to nip bad habits in the bud!)
I was successful starting BE after about six months coming back because I had been practicing 2 to 3 hours a day for about five months. (Yes, I'm very compulsive!) I had a very consistent, although range & endurance limiting embouchure before I started BE.