rocker — I don’t think this is an issue of “ok” or “not ok”. It is simply a sensitivity. Some people feel that this is one way of avoiding too much informality between men and women who are not related to each other. I feel it is even more of an issue when dealing with a spouse’s friend, as it is very possible you come in contact with them frequently and in friendly settings. By avoiding using the first name you remind yourself that your wife’s best friend is not your good friend — she is an unrelated female (or vice versa).
Just to make it clear — I don’t think there is anything “wrong” per se — it is just a sensitivity that I believe many in the “yeshiva world” try to adhere to.