Honestly, it would probably stop and make me go "Okay, odd, but Ooke". Thing is, DP and I are two seperate people. Our thoughts and feelings are our own. I trust him completely that if it's something I need to know, then I will. If not, then he's free to do as he likes. Just because we're in a partnership doesn't take away our individual rights to privacy, whether it's a man or woman. Our friends dont have genders, theyre just friends and if I started treating my DP like he had no rights to privacy and personal conversation, then there'd be no point in us being in a relationship.
Feelings are always valid, theyre feelings and you have the rights to them, but before everyone in this thread started saying there might be something more sinister going on, did you think there was? If YOU thought there was, originally and with no outside influence, then there might be, but otherwise I think that how you react to this situation may end up damaging your trust in your partner, just from the inkling of doubt placed there by other people.
If he's always had female friends and he's never given you a reason not to trust him, then why are you now?? It may be that since she's separated from her husband, she needs someone to talk to and if your husband and her are closer than you and her are, it may just be she finds it easier to talk to him than you and she trusts him that HER thoughts and feelings won't be shared with all and sundry.
My best friend is also my ex fiance and I trust that what I say to him stays with him, and he doesn't tell his current DP despite them living together with a baby on the way. The reason this is is because I'm entitled to privacy for my thoughts and feelings, and he and his DP are two seperate people. I would be very unhappy if I found out that it went beyond him and I because she demanded to know everything I do, say, think and feel and would promptly end the friendship.