He needs to talk to an attorney, not the girl (who can testify against him regarding his statements), and certainly not the person whose job it is to convict him. His attorney will have a duty to protect his legal interests; why on earth would he want to talk to the two people who are acting against that interest.
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Grand larceny is generally a crime that means that a person deprives someone of property, in this case a phone, with the intent of permanently depriving the person of it. In this case, throwing it in the water could certainly be considered as evidence that he intended to deprive her of it. Grand larceny also requires that the value of the item be in excess of $200. I suspect that most cell phones or smart phones would qualify and the court will likely determine value based on the cost to replace it. Sometimes, if the phone is replaced, it could be considered mitigating evidence and possibly result in a reduction to a misdemeanor or even a dismissal all together of the charge.
He should not contact the girl nor the prosecutor. This person is charged with a felony offense, although honestly it sounds more like misdemeanor destruction of property (assuming the phone is worth less than $1000). He should consult with an attorney in the Richmond area and follow his/her instructions. Hypothetically.