Depends how hold his girlfriend is, but assuming she's a teenager, it sure is illegal.  Dissemination of child pornography will get you about six months with lengthy probation in Suffolk County--and the State Police are constantly scanning.  Tell him, in no uncertain terms, to stop.  I think you probably will get away with this one--but if it continues, they'll track you down in no time.  I'm not sure if the State Police have access to the actual content of text messages--so you should be safe.  But if he's emailing them, he's going to get into trouble.