It is not unusual in criminal trials, and sometimes even in civil cases, to "sequester" witnesses, meaning the witness is not allowed in the courtroom in advance of his or her testimony. In criminal cases, it is not uncommon to extend this to a prohibition on the prospective witness learning what other witnesses have said by reading or watching news reports, or by reading a transcript. That does not mean a lawyer can't prep the witness. The lawyer simply cannot do so by telling the witness directly what other witnesses have said.
That said, Brinkema is raising a tempest in a teapot, and I don't doubt for a moment that her ideological bent has something to do with it.
If the lawyer was giving the witnesses trial testimony, that's obviously wrong. This is especially true if it was testimony from the punishment phase, which is what this is all about.
Still, the judge should inquire to determine whether the testimony has been tainted, not just exclude them.