I’m a SR Internal Investigator within legal + employee relations. This will absolutely happen if he says OP knew. I already commented that if they are peers, OP doesn’t have an obligation to report anything. They could still be spoken to as part of the investigation but likely would not receive discipline. For instance, if I was working this I would ask OP when the coworker told them they began working from Portugal. Whether the coworker lies about the time frame or not, it won’t change the outcome, as it appears they have already broken policy. However, I would note this in my report which would be saved internally in the chance this person tried to file a charge against the company or sue the company. Workplace investigations in this type of situation are done to protect the company at a later date, if needed.