No she absolutely was not right in what she did, what she did was appalling.
In such circumstances, a teacher should have been informed of the offenders actions, and the problem dealt with. She should not have take the law into her own hands, but if she did she should have applied a measured response - such as perhaps a slap.
This fella was wrong to do what he did, but he did not deserve to go through such sever trauma, and be left sterilised.
Look at it from another perspective, no matter what a girl did in school, she should not deserve to be kicked so hard in the abdomen that she should be left in agony, with permanent damage to her womb and unable to have children. Would that be fair? Hardly, so why the double standard?
Not only should your sister be expecting ensuing legal action, but she deserves it.