My girlfriend and a dick face, what should i do?

[Andrew]

I'm a sixteen year old junior and my girlfriend is a seventeen year old senior. I've known my girlfriend for four years, but we only recently started dating two months ago. She told me she loved me on Christmas and sincerely meant it. I'm knew i loved her too. I have had other relationships before, but none as serious and dedicated as this one. About two weeks ago though, information had been leaked on twitter by "dickface" that my girlfriend (at the time, we had been dating for three weeks) and some friend of hers that she knew in high school, were at a party, at which they drank a bit and shared a "three second kiss". (i was in mexico at the time visiting my family by the way.) my girlfriend then immediately left the party, but did not tell the friend of hers that she had a boyfriend and shouldn't have kissed him. she kept this a secret from me and continued the relationship as if nothing happened. Two weeks after the scandal she tells me that she loves me, and at the moment, i knew i loved her too (i still didnt know about the party incident). our relationship then grew and prospered, but two weeks ago was when the information was leaked by "dickface" (who is not the same person as the one she kissed). on twitter, "dickface" called my girlfriend poor, slutty, a whore, called her mother names and led multiple attacks on her in one night. my girlfriend called me in the middle of that night to tell me everything that happened (at that moment, it had been five weeks since the incident). she pleaded with me not to break up with her because she loved me and knew that if she had told me so early in the relationship, we wouldn't be where we are today.

The story of the incident differs slightly.
Girlfriend- one second kiss that he initiated after having a couple drinks, which she abruptly ended and told him (whom she claims she has no feelings toward) that she had a boyfriend. she then left the party.
The guy she kissed- three second kiss and afterword she said nothing to him about having a boyfriend, but just left the party because her ride was leaving.
Tonight was the first time i had ever talked to this guy, i was polite and told him there were no hard feelings and he apologized and claimed that he had no idea that she had a boyfriend. he said he was sorry about what happened AND said that if he was in my shoes, would have ended things with her.
I have no idea who to believe and no idea what to do.

Second problem, dickface is talking about her again, at school and on twitter. how should i approach him?

[Tasha]

Don't. Just let it be.

[Swamp Gator]

What has been done is done. The past is the past. Today is today and the future is yet to come... Put the past behind you and live in today looking forward to the future.

Let it go and don't try to figure it out, the facts is, for what ever reason she left the party and is still with you. If you allow this small thing to affect your relationship with her, how do you plan on handling the bigger problems you will face in your future? You both care about each other and that should be enough unless there starts to be a habit/pattern of these sorts of things.

You should tell her though that honesty and trust is very important in a relationship if you want it to be a lasting one. In the future you want to hear things that could upset you or effect the relationship from her not someone else.