From the question it is difficult to tell whether the trial is over or has not yet begun.
If the trial is over:
If anything stated by the witness was material to the guilt of Donald, then the government has a duty to disclose the identity of the informant. Because the informant's relationship with the police can be used by the defense to cast doubt on the honesty of the informant's evidence, thereby providing Donald with exculpatory evidence that can be used to disprove, beyond a reasonable doubt, his guilt. Moreover, the prosecution has a duty to disclose the identity of the accuser to the defense and permit the defense to face his accuser in court.
The US Supreme Court has held that evidence that is materially exculpatory, such as revealing the relationship an informant has with the police (thereby casting doubt on what the witness says), which can be used to prove innocence of a defendant, must be revealed in court. Further, in trial the prosecution has a duty to disclose all evidence it has to the defense. See Banks v. Dretke, 540 U.S. 668 (2004) and Brady v. Maryland, 373 U.S. 83.
If the trial has not yet taken place:
Donald has a right to the identity of his witness. Under the US "Confrontation Clause" a defendant has a right to conduct cross-examination. The US Supreme court has stated that this right is satisfied only if defense counsel can question a witness on any proper subject of cross-examination. See Pennsylvania v. Ritchie, 480 U.S. 39 (1987). Because the informant is the sole witness, defense counsel clearly has a right to cross-examine this sole witness to prepare a valid defense.
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