Personally, I wouldn't mention the taping. I would just say "You and OM seem to close and do too much together. It makes me uncomfortable and insecure in our R. I would like the R to change". You don't need to accuse her of anything, or show her proof of anything to start changing the dynamics of the relationship.
She will probably tell you, that you are being silly and misinterpreting the situation. Don't argue with her, just restate what you feel, end your relationship with the OM and start DB'ing. Let her sort out her feelings for the OM and for you.
Just my $0.02 cents. Of course, given my current M, I am not sure I would take advise from me.
