What should I do in this 'friendship' situation?

READ DESCRIPTION before you attempt to answer…

Okay,
I know this girl – shes beautifull, amazing, great to be around, funny, not the ‘slutty’ kind. Ive known her all my life – im close to her, our parents are close (by the way im male and were both 15).

Today, I was out with a girl in my friendship group and she had no idea i was close to this girl (lets just call the first girl Abs) and she told me Abs had been sleeping with a lot of guys from the year above – at first I refused to believe it untill more people backed up the fact and I pretty much knew it was more dangerous and serious than a rumour and ive been incredibly mad at her in my head and VERY disappointed because shes not in the slightest ‘that girl’ and the thing is I want to express to her my fealings in what shes done and that shes been a complete idiot – but ive swore to my ‘source’ that i cant mention it to anyone because they really shouldn’t have told me and they only did because of how close i am to Abs and i cant bring it up to Abs as if its not a big deal and if it is all rumours – which I doubt as its apparantly a huge thing and she turns around and says its all not true – i look and feel idiotic! I also believe its her friendship group that have made her do this sort of stuff as they’re like that…

So please give advise on what to do – what you’d do – no smart ass comments like – “talk to her”