My best friend (of several years) is female. And it is not my wife. My wife actively encourages this friendship, and actually believes the influence of said friend has improved our marriage. The husband of my female best friend is also a friend of mine, and says that I have been a positive influence for their marriage too.

To sum up: The idea that men and women cannot be best friends isn't true for me. A lot of people disagree, but I am inconvenient evidence that it can and does work, when both parties are truthfully platonic.

I'll leave you to extrapolate my opinion on the situation described herein.