What is the ruling on the testimony of a person who heard the events from the plaintiff, knowing that the witness was his colleague at work and that his failure to testify may lead to the loss of the plaintiff's right?

Testimony requires two things: the truth of what is being testified to, and the witness's true knowledge of it. Allah says: "And we did not testify except to that which we knew." Therefore, it is not permissible to testify merely upon hearing a complaint, because that is not sufficient for acquiring true knowledge of the matter. However, if the truthfulness and trustworthiness of the complainant are known, then it is permissible to testify for him based on what is known.