It is also enshrined in Rule 5.1-2(e): "When acting as an advocate, a lawyer must not knowingly attempt to deceive a tribunal or influence the course of justice by offering false evidence, misstating facts or law, presenting or relying upon a false or deceptive affidavit, suppressing what ought to be disclosed or otherwise assisting in any fraud, crime or illegal conduct."
And it is reinforced by Rule 5.1-2(i), which prohibits a lawyer from deliberately refraining from informing a tribunal of any binding authority that the lawyer considers to be directly on point.
Legal Maxim: "Fiat justitia ruat caelum" - translation - “Let justice be done, though the heavens fall.”
The Duty of Candour is absolute. It admits of no exceptions. A lawyer may not mislead the Court, even if their client instructs them to do so. A lawyer may not suppress authority, even if it undermines their client's position. A lawyer may not make representations they know to be false, even if they believe the Court will not discover the truth.
In Groia v. Law Society of Upper Canada, 2018 SCC 27, the Supreme Court of Canada confirmed that while a lawyer must fearlessly advocate for their client, that advocacy is entirely constrained by the Duty to Act with absolute integrity, honesty, and Candour before the tribunal.
The Court stated: "A rigid definition of when incivility amounts to professional misconduct in the courtroom is neither attainable nor desirable. Rather, determining whether a lawyer's behaviour warrants a finding of professional misconduct must remain a context-specific inquiry that is flexible enough to assess behaviour arising from the diverse array of situations in which lawyers find themselves."
However, the Court also made clear that there are limits. Allegations that are made in bad faith or without a reasonable basis amount to professional misconduct. A lawyer cannot cross the line from zealous advocacy into misrepresentation.
The Court further stated: "Allegations that are either made in bad faith or without a reasonable basis amount to professional misconduct."
Groia v. Law Society of Upper Canada, 2018 SCC 27 (CanLII), [2018] 1 SCR 772:
The irony here is that you cannot sue a lawyer who is not your lawyer by contract, and you can only sue your lawyer for negligence.the Appeal Panel developed an approach that is both flexible and precise. A rigid definition of when incivility amounts to professional misconduct in the courtroom is neither attainable nor desirable; rather, determining whether a lawyer’s behaviour warrants a finding of professional misconduct must remain a context‑specific inquiry that is flexible enough to assess behaviour arising from the diverse array of situations in which lawyers find themselves. Yet, standards of civility must be articulated with a reasonable degree of precision. The Appeal Panel’s approach strikes a reasonable balance between flexibility and precision: it sets a reasonably precise benchmark that instructs lawyers as to the permissible bounds of ethical courtroom behaviour, by articulating a series of contextual factors — what the lawyer said, the manner and frequency in which it was said, and the presiding judge’s reaction to the lawyer’s behaviour — that ought generally to be considered when evaluating a lawyer’s conduct, and by describing how those factors operate when assessing a lawyer’s behaviour.
No case has ever been successful yet in Canada of suing an lawyer that is not your lawyer, no matter what they have done.
There is no rule against it, that would be blatantly unjust, it just has never happened yet.