Some men truly don’t recognize that their friendships with women cross over into inappropriateness. Most of the times, it becomes terse, dry and arid. You have to moist your relation with the steam of love and care.
Try saying something like, “I love the relationship that we have and I trust YOU, but I am uncomfortable with how close you are to ….”. usually our response becomes much more accusatory causing our partner to become defensive. Your partner will not fine you, first palatable then, tolerable.
Don’t make your mate feel sorry; protect his/her honor as you do it with yours.
Your partner may truly see nothing wrong with its special relationship, especially if it truly has not developed intense romantic feelings toward the outsider. Explain your partner calmly that his/her motives behind the friendship are not in question, but that his/her behavior (spending one-on-one time with the outsider, confiding in one another, not integrating you into the friendship) is inappropriate for someone in a committed relationship and it needs to be changed.