The Ethics Panel empathizes with your predicament. No one wants to be in the position of policing a colleague. That said, it’s important to keep in mind that the conduct of a colleague–particularly a teammate–may call into question the integrity of the team and reflect badly on the profession. The interpreter you describe is violating GR 11.2(4) (See GR 11.2, Impartiality and Neutrality), which states explicitly that “[Interpreters] must refrain from conduct that may give an appearance of bias . . . “ (italics added).
Should you decide to bring up the issue yourself–which is permissible, though not required–it may be helpful to have a clear understanding of the importance of avoiding any appearance of bias, so that you can better explain it to your colleague (See Standards of Practice, Perceived Conflict of Interest (Appearance of Bias)). Indeed, addressing a colleague’s errors, be they in interpretation or conduct, requires great tact and professionalism (See Standards of Practice, Errors by Colleagues). You might consider saying something like, “I’m concerned that your conversation with the defendant may make our team look biased” or “I know you may be talking about the weather, but other people don’t know that and may assume otherwise.”
A case from 1996 provides an instructive example of the possible outcome of a conversation that initially may have seemed harmless. In that case, the defendant engaged the interpreter in conversation. He told the interpreter about his and the co-defendant’s involvement in the case. The results were as follows:
The judge considered declaring a mistrial.
The interpreter was almost called as a witness in the case.
The interpreter was replaced for the remainder of the trial.
The attorney stated emphatically on the record that no interpreter was to have any conversation with his client out of the presence of the attorney.
(See KCOIS letter of April 2, 1996)
While the behavior you describe may not rise to that level, conversation with the defendant opens the door to unforeseen consequences, as well as creating the appearance of bias.