It will depend on the law of privilege in a particular jurisdiction. In some places, the privilege is an evidential one: it’s just that neither the lawyer not the client can be forced to disclose the legal advice given, but if the advice is known from some other route, it may be admissible.
But in other places, the privilege is more substantive law: the advice given is inadmissible in court, no matter the source. If the jurisdiction where the events occured has the substantive approach, @Horatius may not be able to testify, even if he’s not the holder of the privilege. Laws vary.
To take Horatius’s hypothetical, it may also depend on why they were in the room with the other person and the lawyer. If Horatius is a separate client of the lawyer, it could get very complicated; who is the lawyer giving the advice to, and why are two people there? This is why a lawyer doesn’t normally give advice to two different clients at the same time, unless there is a clear co-counsel agreement between the clients, addressing the privilege issue.
If Horatius is there because they’re an employee of the person getting the advice, then the situation is clearer: Horatius may equally be bound by the privilege, because of their employment relationship. The privilege would not be of any force in a large operation if any employee is able to waive the privileged information. A large operation receiving legal advice needs to be able to keep it confidential, even if the advice was not given directly to a particular employee, and will generally be protected by the courts.
That doesn’t mean that everything that is said in a meeting with a lawyer is privileged; it’s not a Cone of Silence. It’s only the legal advice: the request for advice by the client, the disclosure of facts that underlie the request, and the advice back from the lawyer. But comments from other people in the meeting, unrelated to the legal advice and which don’t disclose the legal advice, likely would not be covered by privilege.
So suppose you have this conversation, with only three people in the room:
Big Boss: "Lawyer Jane, thanks for coming. Horatius here is my trusted employee and familiar with the file. I want to do Operation X. [Detailed outline of facts follows]. Is that legal?
Lawyer Jane: “No, Mr Boss, that’s not legal at all. I strongly advise you against doing it. You could be in a world of trouble. You would probably be committing a crime [detailed legal advice follows].”
Big Boss: “Thanks for the advice. Please send me your bill.”
[Lawyer Jane leaves the room.]
Big Boss: “Horatius, I want you to do Operation X - set it in motion, right away.”
Horatius: “Boss, I’m really uncomfortable with this proposal. I don’t want to do Operation X. We’ve just heard from Lawyer Jane that it’s likely illegal.”
Big Boss: “I don’t agree with Lawyer Jane’s advice. Start putting Operation X into play, as soon as you leave this meeting.”
Horatius: “I quit. I’m not doing something illegal. Find someone else to do Operation X.”
Horatius later finds out that the next one in line in the office did start up Operation X for Big Boss. Horatius goes to the police. Big Boss gets charged after an investigation. Here’s what Horatius would likely be able to testify to:
Big Boss: "So, Lawyer Jane, thanks for coming. Horatius here is my trusted employee and familiar with the file. I want to do Operation X. [Detailed outline of facts follows]. Is that legal?
Lawyer Jane: “No, Mr Boss, that’s not legal at all. I strongly advise you against doing it. You could be in a world of trouble. You would probably be committing a crime [detailed legal advice follows].”
Big Boss: “Thanks for the advice. Please send me your bill.”
[Lawyer Jane leaves the room.]
Big Boss: Horatius, I want you to do Operation X - set it in motion, right away."
Horatius: “Boss, I’m really uncomfortable with this proposal. I don’t want to do Operation X. We’ve just heard from Lawyer Jane that it’s likely illegal.”
Big Boss: “I don’t agree with Lawyer Jane’s advice. Start putting Operation X into play, as soon as you leave this meeting.”
Horatius: “I quit. I’m not doing something illegal. Find someone else to do Operation X.”
By this approach, Horatius is able to testify to his conversation with Big Boss, so long as nothing he says discloses the legal advice given by Lawyer Jane, in response to the request by Big Boss for legal advice.
Highly simplified, of course, and simply a hypothetical for discussion purposes on a matter of public interest. Not meant as legal advice. Laws vary considerably from place to place, and I’m just some shmoe on the interwebs.