A document indicating that a prison guard allegedly encouraged an inmate to commit suicide, though misfiled by an attorney and unseen for almost a year, should have been allowed in a mother’s lawsuit against the prison, a federal appeals court has ruled.

The U.S. Court of Appeals for the Third Circuit overturned a New Jersey federal judge’s decision to deny plaintiff Joan Mullin’s request to revise her mostly dismissed lawsuit to include the new evidence.

Mullin sued members of the New Jersey prison system over her son Robert’s death by hanging in his cell. Two years into the litigation and unbeknownst to Mullin, her attorney unwittingly came across an investigative report that said six inmates heard a guard referred to as “Officer X” deny Robert’s pleas for psychiatric help and tell him he “might as well kill himself,” according to Nov. 6 opinion by Third Circuit Judge Julio Fuentes.

Mullin’s lawyer, Shelley Stangler, misfiled the new discovery and was unaware of its contents. When she realized her mistake, it was too late and the judge denied her motion for leave to amend the complaint.

However, Fuentes said the judge’s decision was an “impermissible exercise of discretion.”

“Some of the factors relied upon to deny leave are not supported by the record or are at odds with our case law,” Fuentes wrote. “And while we do not intend to minimize counsel’s mistake, it does not, standing alone, support denying leave to amend.”

He added that since Mullin did not know of the new evidence until later, it would be improper for the statute of limitations to start running before that point.

“Mullin’s investigative diligence has not been called into question. Thus, either innately or through the application of the discovery rule, the facts of this case show that Mullin could not have learned of the particular nature of this serious disregard of Robert’s mental state—or who was at fault—until she had obtained the report or its equivalent. Mullin’s attempt to amend therefore fell well within the applicable two-year limitations period if measured from the April 2013 disclosure date,” Fuentes said.

Reached Monday, Stangler, of Springfield Township, said, “My sense of the decision is that the Third Circuit kept the interest of justice in mind and supported it with the law.”

“A mother lost her son,” she continued, “and it was a very compelling case. A clerical error could not subvert the interests of justice.”

Christine Kim Neeman of the New Jersey Attorney General’s Office, who represented the state defendants, did not return a call or email seeking comment on Monday.

P.J. D’Annunzio can be contacted at ­215-557-2315 or Follow him on Twitter @PJDannunzioTLI.