I have not experienced the type of financial infidelity that your friend has. But I think many of us here have seen a bit of finical infidelity that goes along with sexual infidelity. How often doe we see people post about bank accounts that have been drained or credit cards that have been rung up that the BS did not know about. I experienced some of that myself.
This site may be too specific about sexual infidelity to help your friend, but then there may be many things that ring true for her -- the lies, trickle truth, gaslighting, and blameshifting. So maybe it would help, if nothing else to know that she is not alone and she is not crazy.