As a friend, it is your duty only to warn, not to direct anyone's actions. If she's a good friend to you, you could try telling her that you have a few doubts about this guy (without getting into the gory details), and that she should be a little cautious. Do mention that this guy has been your friend for a while, so that it gets into her head that friends are supposed to know what each one's been doing
Either way, don't get too serious about it (say it all in a light hearted manner) - it will get
you into trouble rather than anyone else. And don't go out of your way trying to get them broken up. You must understand that ultimately it's her decision to break up, and she will do it
when she wants to (this is
especially true of teenage girls!).
And finally, don't bother too much about it.....keep yourself going strong - it's often worthless to spend time over things you don't have direct control of. Let things naturally take their course. Girls are smart too, they've got secrets too, as well as a "gossip circle".....now if the three of you have common female friends, stuff will flow around and eventually the girls find out too (the "gossip circle" is a big theory I don't want to get into right now

).
So, you just take it easy and enjoy your life. Warn her, advise her, but do NOT even try to compel her to do anything.