Should she testify to something that she has not been asked for but that may benefit the person against whom she is testifying?
Question

                I work in a department and it so happened that one of the men insulted the head of the department. I was present with a group of other people, so they asked us to testify. I will testify and – in sha Allah – I will tell the truth which is that he – may Allaah guide him – insulted her openly and in front of her; he cursed her father and her. After he went out, when they dragged him out, she said: “I’ll show you, O So and so,” i.e., a threat – but he did not hear her, so he did not say that she had said, “I’ll show you,” because he was nowhere near the desk then. I was the one who was nearby and heard it. Should I say that when the matter is investigated? I hope to receive a reply.
Praise be to Allah.
    This phrase (“I’ll show you”) is a threat but what may be 
    meant by it is to make an official complaint, which is in fact what 
    happened. We do not think that it matters if you tell them or not when the 
    matter is investigated. 

    If she added any other words to it, such as insults or 
    condemnation of the one who insulted her, then you should mention that, 
    because she may have taken her rights by saying these words or more, and it 
    is not right to punish him when the other person has already settled the 
    score. Responding to an insult in kind is a kind of taking one's rights or 
    settling the score, as al-Nawawi said in his commentary on the words of the 
    Prophet  (peace and blessings of Allaah be upon him): “When two people 
    trade insults, the sin will be on the one who started it, unless the one who 
    has been wronged went too far in returning the insults.” Muslim 4688. 
    Al-Nawawi said: They said:  If the one who has been insulted return the 
    insult then the matter is settled, and the first one owes nothing to the 
    second one, but the sin of initiating the insults remains on him. And it was 
    said: He will be free from any sin if the other person returns the insult 
    but the blame will still be on the initiator, but not the sin.  

    If what was said in response is not as bad as what was said, 
    then the matter is not settled, however by responding, part of the matter is 
    settled. This may remove or reduce the punishment for the one who insulted 
    the other first. 

    Islam commands us to be just with all people. Allaah says 
    (interpretation of the meaning): 
“and let not the enmity 
    and hatred of others make you avoid justice. Be just: that is nearer to 
    piety; and fear Allaah. Verily, Allaah is Well‑Acquainted with what you do”

    [al-Maa'idah 5:8]

    And Allaah knows best.