Cyber-sex is not sex and the age of consent is not the issue. In fact in NY at least a 18 year old and 15 year old can have sex and it isn't a sustainable crime, but if the 18 year old has cyber sex with the 15 year old, he is guilty of both Federal and state crimes in both her state and yours. Again if they aren't at least 18, stay far away.
This answer does not, nor is it intended to, create an attorney-client relationship or constitute attorney advertising. Rather, it is offered solely for information purposes. Since the facts of each case are different, it is critical to consult with qualified counsel with whom information can be shared and assessed under an attorney-client privilege, so that competent advice can be obtained on which you can make informed decisions.