According to most maraja' it is makruh for a woman to talk to a man unless it is necessary. Often, a woman can't see a situation how a man sees it. The woman may believe that the non-mahram she is talking to sees her as a genderless colleague, but most men, when talking honestly, say that men are usually conscious that they are talking to a woman. The male friend obviously enjoys talking to your wife, otherwise he would not do it. He does not realise that have a close, or familiar friendship with another man's wife is highly disrespectful to that man and in a sense invades the sanctity of the home. Islam encourages men to have ghirah; to have a sense of protecting their family and guarding their family's honour. In some countries this understanding has largely been lost, so that men have become silent and pacified. These cultural issues are very delicate and difficult to explain. I don't know if you could put the scenario to her of your supposedly having a 'female friend' that you talk to regularly, and ask how she would feel about that. Most women feel strongly about their husbands having other women around them. How would she feel if you had a female friend that chatted freely with you, but was not her friend? While this is not an Islamic argument - more an argument to appeal to a Westernised mind - it may be an argument that can appeal. In addition, this narration is in Sunni sources: "The Prophet (s) said: "Their (husbands') rights over you (wives) is that you do not allow anyone whom they dislike onto your bedding and you do not allow anyone whom they dislike into your house." [At-Tirmithi]". I haven't managed to find an exact equivalent in the Shi'i texts, but it may be worth searching for it.