Suppose that instead of the secret cell phone tracker evidence and the parallel construction, it was an undercover witness and a random bystander witness. Inste

Suppose that instead of the secret cell phone tracker evidence and the parallel construction, it was an undercover witness and a random bystander witness. Instead of having the undercover officer testify in court and thereby reveal themselves, the cops instead use the undercover witness's information to locate the bystander (e.g. the undercover witness remembered seeing the bystander and gave the police his description to help them find the bystander), and they have the bystander testify in court instead of the undercover witness, and they never say at the trial how they found this witness. Are the police allowed to do this? Should they be? If they are, is it any different from "parallel construction"starting from secret cell phone evidence instead of secret witness testimony? (I'm genuinely curious. I don't know the answers to any of these questions.) Edit: I guess in order to make the analogy more accurate, I would have to specify that the undercover agent knows they did something illegal that would make their own testimony inadmissible as evidence, and this is the motivation for finding the bystander.