My girlfriend was d by my best mate HELP ?!?!?!?!?!?!?!?!?

I made this coz i need ur help. im 14 yrs and my girlfrind is 13 there is not really any age between us. but a will back her, me, and my best frinds got very drunk for a birthday ( we never do btw ) my best mate came on to her but she just pushed hit away then came and told me coz we never keep any think from each outher,.when i found out i got into a fight with him and both ended up getting hurt like cuts all that,. he said he would get his own back one way or a nother,. so it was about 3 wks later and my gf came round mine crying her eyes out :'( she told me she had been d i asked who by ad it was my best mate said it was all true i i belive it was coz she would NEVER make it up.then he sent it round the skl she has had some think like this befor coz some how it got out that we wear sleeping togethor (btw we r ) i really worried about her now coz she dosent eat or sleep or do any think ,. my mum & dad dont know ad hers dont ,. so can u plz help me i want to help her so much i miss the old her .xx <<< kisses 4 her :)