If I knew what you know about past incidents and my friend was talking to me about her doubts I might say, "Sometimes kids lie, I know mine have." And just leave it at that for her to think about. You should be able to be honest with her in a way that makes her feel it is not a major catastrophe, just a behavior that needs to be addressed for her daughters sake. You sound like a conscientious friend who wants to do the right thing.

If I were your friend I would want you to tell me your honest opinion and discuss thoughts about what's going on.

In other words, with all the hinting around being suggested, one thing to be hinted around might be WHY the kid is afraid to confess her involvement in these incidents.