Was anything put in writing at the time of the incident? If it goes to disciplinary, there will need to be evidence of the incident (whatever it was), to show that your friend did whatever it was. Any witnesses, either to the event or to the employer's statement that it wasn't important at the time?
Is your friend in a union?
If there is nothing to support either side, I suggest (as I often do) that they ring the ACAS helpline - impartial and free, and very helpful and supportive.