Praise be to Allaah.
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”
And Allaah knows best.