You are absolutely correct to have these concerns whether the girls parents know about their friendship or not. If an allegation arises of any type of sexual conduct your son can be charged as an adult with a sex crime and if convicted faces not only incarceration but a virtual lifetime of not only being labled a sex offender, but being legally required to continuously register with the police as a sex offender. Also, please keep in mind that all that is required for your son to be charged is to have the girl, or her parents, make a complaint that there have been sexual relations. The girl is underage which of course means that her consent is not a defense; she legally cannot consent to have sex. To be charged and tried and convicted all that the police, prosecutor and a jury need is what sounds and appears to be the credible testimony of the girl. The law doesn't require corroboration of a sexual assault victim's testimony. Lastly, considering what is usually the unstable nature of "young love" all your son has to do is make this young lady angry, break-up with her or some other trivial thing and it can lead to a lifetime of trouble and problems for your son.