Wow. That is awful. You say the criminal has "admitted" to the charges, but has he plead guilty? I ask this because the only way your visit could have any effect is if he pleads not guilty and your girlfriend takes the stand. The attorney at this point may, unfortunately, try to paint a picture of a promiscuous woman falsely accusing the criminal of rape because she doesn't want her boyfriend to find out. I doubt this would work, but it happens. The plain fact of the matter is, the investigators have two people who say this occurred as well as a bullet in the wall. Big time evidence.
As far as sentencing goes, none of the things that you mention should really make any difference: virgin or no virgin, boyfriend or no boyfriend, etc. Rape is rape, whether it is against a virgin by a stranger or against a wife by an estranged husband for that matter. Hopefully you and this young woman's family can work out this private matter amongst yourselves, and hopefully this guy spends the rest of his life in Starke.
I cannot imagine a judge, in a case as big as this, being impressed by the victim's boyfriend paying a visit after the crime.