Legal Question in Criminal Law in Virginia

It sounds like the lawyer is trying to do what is in your brother's best interests, by helping him make the best of a bad situation. The way things go in federal drug cases, striking a bargain quickly is often an important part of that, because doing so has a significant effect on the sentencing guidelines.

Your brother's lawyer is correct, i.e., once a defendant elects to enter plea negotiations with the prosecutor, he's at a procedural fork in the road in his case, so to speak, and necessarily must forego the path of further discovery and continue forward on that leading to the plea of guilty; the two are not intended to run simultaneously along parallel paths since each has a purpose/objective which is distinct and substantively different from the other.

If the defendant and his attorney initially believe that they may have a defendable case at trial, they should not enter into plea negotiations until all appropriate discovery (whether formal or informal)has been concluded and that as a consequence they are in a reasonably good position to fully evaluate the strength of the government's evidence.