First - As a function of the limited forum here, there is just no way that you can provide all the facts and circumstances to be taken into account when advising someone to accept or reject a plea offer.
Second - there is no way someone here can second-guess your brother's attorney.
Just from the facts you gave, your brother COULD be charged as an accessory to the drug deal.
If you have doubts about the advice, ask the lawyer to walk you and/or your brother through the pros and cons of accepting the offer and pleading guilty. He/she has no obligation to explain it to you, but many attorneys will anyway. Ultimately, it is your brother's call.
This answer is provided for informational purposes only. It does not constitute legal advice, nor does it establish an attorney-client relationship. Speak to an attorney in your area to protect your legal rights.
Whether the decision to plead guilty to the F3 is right or wrong is difficult to answer without the benefit of knowing what was done to reach that recommendation by your brother's attorney. I certainly feel, at a minimum, your brother should feel comfortable with the effort his attorney has committed to the case and that there was sufficient attempts to negotiate a more favorable plea deal if going to trial was not in his best interest.
This is a general answer to your question and does not create an atty client relationship