chips fall where they may. Depending on what your agreement(s) say, if you think the fraud is big enough that the police might need to be involved, you might consider tipping them off but truthfully they won’t normally get involved in such things.

You could also I guess encourage C to do some kind of audit of all its relationships designed to uncover frauds like the one you know about, but in the end I don’t see how you can tell C anything without breaching your express or implied obligations to B.