Let her know that you are willing to be fully transparent with her with regards to this co-worker, and ask if she's willing to do the same with her OM.
She can check your cellphone anytime she wants to. Give her your e-mail passwords, home, work and Facebook, as applicable. Invite her to meet you and female co-worker (and her husband, if she's married??) for a drink sometime. Let her know that you believe that a married person shouldn't have any friends that aren't SUPPORTERS of the marriage.
