I should think telling your friend would most likely result in him not believing you and very probably getting very angry at you. Sorry to say so, but that's invariably how it ends... Maybe he will find out for himself later, but if you're going to tell him, prepare to be cursed at and called liar.
What *I* would do, is confront the girl, and tell her SHE needs to tell her boyfriend. Tell her that if she won't do it, you will. This threat usually works. Otherwise, I did find 250's suggestion about taperecording her pretty good actually. It may be a bit mean to tape someone when they aren't aware, but it would be great as a back-up plan in case your friend won't have it.
------------------
Melusine, Sultry Elflet,
High Queen of Fluffies,
Archbabe of the OHF,
LH, HADB and ORT!
Your voice is ambrosia
Amy Brown Fantasy Art