My son's gf reads his cell phone messages and accuses him of cheating on her. She also calls phone numbers on his phone to see if a female answers.

The short version based strictly on the information you supplied is "probably not."  Much depends on how she got the phone (for example, if she committed a burglary to steal it, that is another matter.  Or if she hacked into his phone (or any otehr computer); you did not say anything about password protection).  But -- at least today and by itself -- reading messages on someone else's iPhone does not appear to be a criminal offense.