Are you meaning to say that now your son is SI-ing or she is cutting him in some way? Or do you mean that your son is emotionally affected by seeing his girfriend do this to herself?
If the first is what you meant, then that it definitely not normal, and your son needs to know that no one has the right to hurt him like that, even if they are hurting.
If the second is what you meant, then there's some articles on the main website that might help him (and you) better understand what SI-ing is about and why people do it.
http://www.recoveryourlife.com/index.php?categoryid=68 <--that link contains articles on why people injure themselves.
http://www.recoveryourlife.com/index.php?categoryid=115 <--this link is the self-injury FAQ, which may answer some more of your questions.