If an incident occurs, approach the situation carefully and calmly. Do not over-react as this may make your child back off and deter them from confiding in you again.
Get your child away from the computer for a while.
Watch some TV, have a cup of tea, or just get out for some fresh air. This will give both of you time to calm down and gain some perspective. ‘Assess’ the threat to your child as objectively as you can.
Children regularly fall out with each other and arguments are not of themselves a sign of cyberbullying. So talk calmly with your child and find out exactly what you are dealing with.
If there is a problem, reassure your child that you are on their side and will work with them to deal with it.
Encourage your child to find a way to resolve any issues themselves, albeit with your support, before you step in and take over. The last thing your child wants is more drama or conflict so work with them to find solutions.
Keeping a record will help you to prove your case should you need to take further steps. Read here for further technical advice.